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The Elements of Geology by William Harmon Norton

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Both Lepidodendron and Sigillaria were anchored by means of great
cablelike underground stems, which ran to long distances through
the marshy ground. The trunks of both trees had a thick woody
rind, inclosing loose cellular tissue and a pith. Their hollow
stumps, filled with sand and mud, are common in the Coal Measures,
and in them one sometimes finds leaves and stems, land shells, and
the bones of little reptiles of the time which made their home

It is important to note that some of these gigantic lycopods,
which are classed with the CRYPTOGAMS, or flowerless plants, had
pith and medullary rays dividing their cylinders into woody
wedges. These characters connect them with the PHANEROGAMS, or
flowering plants. Like so many of the organisms of the remote
past, they were connecting types from which groups now widely
separated have diverged.

Gymnosperms, akin to the cycads, were also present in the
Carboniferous forests. Such were the different species of
CORDAITES, trees pyramidal in shape, with strap-shaped leaves and
nutlike fruit. Other gymnosperms were related to the yews, and it
was by these that many of the fossil nuts found in the Coal
Measures were borne. It is thought by some that the gymnosperms
had their station on the drier plains and higher lands.

The Carboniferous jungles extended over parts of Europe and of
Asia, as well as eastern North America, and reached from the
equator to within nine degrees of the north pole. Even in these
widely separated regions the genera and species of coal plants are
close akin and often identical.

INVERTEBRATES. Among the echinoderms, crinoids are now exceedingly
abundant, sea urchins are more plentiful, and sea cucumbers are
found now for the first time. Trilobites are rapidly declining,
and pass away forever with the close of the period. Eurypterids
are common; stinging scorpions are abundant; and here occur the
first-known spiders.

We have seen that the arthropods were the first of all animals to
conquer the realm of the air, the earliest insects appearing in
the Ordovician. Insects had now become exceedingly abundant, and
the Carboniferous forests swarmed with the ancestral types of
dragon flies,--some with a spread of wing of more than two feet,--
May flies, crickets, and locusts. Cockroaches infested the swamps,
and one hundred and thirty-three species of this ancient order
have been discovered in the Carboniferous of North America. The
higher flower-loving insects are still absent; the reign of the
flowering plants has not yet begun. The Paleozoic insects were
generalized types connecting the present orders. Their fore wings
were still membranous and delicately veined, and used in flying;
they had not yet become thick, and useful only as wing covers, as
in many of their descendants.

FISHES still held to the Devonian types, with the exception that
the strange ostracoderms now had perished.

AMPHIBIANS. The vertebrates had now followed the arthropods and
the mollusks upon the land, and had evolved a higher type adapted
to the new environment. Amphibians--the class to which frogs and
salamanders belong--now appear, with lungs for breathing air and
with limbs for locomotion on the land. Most of the Carboniferous
amphibians were shaped like the salamander, with weak limbs
adapted more for crawling than for carrying the body well above
the ground. Some legless, degenerate forms were snakelike in

The earliest amphibians differ from those of to-day in a number of
respects. They were connecting types linking together fishes, from
which they were descended, with reptiles, of which they were the
ancestors. They retained the evidence of their close relationship
with the Devonian fishes in their cold blood, their gills and
aquatic habit during their larval stage, their teeth with dentine
infolded like those of the Devonian ganoids but still more
intricately, and their biconcave vertebrae which never completely
ossified. These, the highest vertebrates of the time, had not yet
advanced beyond the embryonic stage of the more or less
cartilaginous skeleton and the persistent notochord.

On the other hand, the skull of the Carboniferous amphibians was
made of close-set bony plates, like the skull of the reptile,
rather than like that of the frog, with its open spaces (Figs. 313
and 314). Unlike modern amphibians, with their slimy skin, the
Carboniferous amphibians wore an armor of bony scales over the
ventral surface and sometimes over the back as well.

It is interesting to notice from the footprints and skeletons of
these earliest-known vertebrates of the land what was the
primitive number of digits. The Carboniferous amphibians had five-
toed feet, the primitive type of foot, from which their
descendants of higher orders, with a smaller number of digits,
have diverged.

The Carboniferous was the age of lycopods and amphibians, as the
Devonian had been the age of rhizocarps and fishes.

LIFE OF THE PERMIAN. The close of the Paleozoic was, as we have
seen, a time of marked physical changes. The upridging of the
Appalachians had begun and a wide continental uplift--proved by
the absence of Permian deposits over large areas where
sedimentation had gone on before--opened new lands for settlement
to hordes of air-breathing animals. Changes of climate compelled
extensive migrations, and the fauna of different regions were thus
brought into conflict. The Permian was a time of pronounced
changes in plant and animal life, and a transitional period
between two great eras. The somber forests of the earlier
Carboniferous, with their gigantic club mosses, were now replaced
by forests of cycads, tree ferns, and conifers. Even in the lower
Permian the Lepidodendron and Sigillaria were very rare, and
before the end of the epoch they and the Calamites also had become
extinct. Gradually the antique types of the Paleozoic fauna died
out, and in the Permian rocks are found the last survivors of the
cystoid, the trilobite, and the eurypterid, and of many long-lived
families of brachiopods, mollusks, and other invertebrates. The
venerable Orthoceras and the Goniatite linger on through the epoch
and into the first period of the succeeding era. Forerunners of
the great ammonite family of cephalopod mollusks now appear. The
antique forms of the earlier Carboniferous amphibians continue,
but with many new genera and a marked increase in size.

A long forward step had now been taken in the evolution of the
vertebrates. A new and higher type, the reptiles, had appeared,
and in such numbers and variety are they found in the Permian
strata that their advent may well have occurred in a still earlier
epoch. It will be most convenient to describe the Permian reptiles
along with their descendants of the Mesozoic.



With the close of the Permian the world of animal and vegetable
life had so changed that the line is drawn here which marks the
end of the old order and the beginning of the new and separates
the Paleozoic from the succeeding era,--the Mesozoic, the Middle
Age of geological history. Although the Mesozoic era is shorter
than the Paleozoic, as measured by the thickness of their strata,
yet its duration must be reckoned in millions of years. Its
predominant life features are the culmination and the beginning of
the decline of reptiles, amphibians, cephalopod mollusks, and
cycads, and the advent of marsupial mammals, birds, teleost
fishes, and angiospermous plants. The leading events of the long
ages of the era we can sketch only in the most summary way.

The Mesozoic comprises three systems,--the TRIASSIC, named from
its threefold division in Germany; the JURASSIC, which is well
displayed in the Jura Mountains; and the CRETACEOUS, which
contains the extensive chalk (Latin, creta) deposits of Europe.

In eastern North America the Mesozoic rocks are much less
important than the Paleozoic, for much of this portion of the
continent was land during the Mesozoic era, and the area of the
Mesozoic rocks is small. In western North America, on the other
hand, the strata of the Mesozoic--and of the Cenozoic also--are
widely spread. The Paleozoic rocks are buried quite generally from
view except where the mountain makings and continental uplifts of
the Mesozoic and Cenozoic have allowed profound erosion to bring
them to light, as in deep canyons and about mountain axes. The
record of many of the most important events in the development of
the continent during the Mesozoic and Cenozoic eras is found in
the rocks of our western states.


EASTERN NORTH AMERICA. The sedimentary record interrupted by the
Appalachian deformation was not renewed in eastern North America
until late in the Triassic. Hence during this long interval the
land stood high, the coast was farther out than now, and over our
Atlantic states geological time was recorded chiefly in erosion
forms of hill and plain which have long since vanished. The area
of the later Triassic rocks of this region, which take up again
the geological record, is seen in the map of Figure 260. They lie
on the upturned and eroded edges of the older rocks and occupy
long troughs running for the most part parallel to the Atlantic
coast. Evidently subsidence was in progress where these rocks were
deposited. The eastern border of Appalachia was now depressed. The
oldland was warping, and long belts of country lying parallel to
the shore subsided, forming troughs in which thousands of feet of
sediment now gathered.

These Triassic rocks, which are chiefly sandstones, hold no marine
fossils, and hence were not laid in open arms of the sea. But
their layers are often ripple-marked, and contain many tracks of
reptiles, imprints of raindrops, and some fossil wood, while an
occasional bed of shale is filled with the remains of fishes. We
may conceive, then, of the Connecticut valley and the larger
trough to the southwest as basins gradually sinking at a rate
perhaps no faster than that of the New Jersey coast to-day, and as
gradually aggraded by streams from the neighboring uplands. Their
broad, sandy flats were overflowed by wandering streams, and when
subsidence gained on deposition shallow lakes overspread the
alluvial plains. Perhaps now and then the basins became long,
brackish estuaries, whose low shores were swept by the incoming
tide and were in turn left bare at its retreat to receive the rain
prints of passing showers and the tracks of the troops of reptiles
which inhabited these valleys.

The Triassic rocks are mainly red sandstones,--often feldspathic,
or arkose, with some conglomerates and shales. Considering the
large amount of feldspathic material in these rocks, do you infer
that they were derived from the adjacent crystalline and
metamorphic rocks of the oldland of Appalachia, or from the
sedimentary Paleozoic rocks which had been folded into mountains
during the Appalachian deformation? If from the former, was the
drainage of the northern Appalachian mountain region then, as now,
eastward and southeastward toward the Atlantic? The Triassic
sandstones are voluminous, measuring at least a mile in thickness,
and are largely of coarse waste. What do you infer as to the
height of the lands from which the waste was shed, or the
direction of the oscillation which they were then undergoing? In
the southern basins, as about Richmond, Virginia, are valuable
beds of coal; what was the physical geography of these areas when
the coal was being formed?

Interbedded with the Triassic sandstones are contemporaneous lava
beds which were fed from dikes. Volcanic action, which had been
remarkably absent in eastern North America during Paleozoic times,
was well-marked in connection with the warping now in progress.
Thick intrusive sheets have also been driven in among the strata,
as, for example, the sheet of the Palisades of the Hudson,
described on page 269.

The present condition of the Triassic sandstones of the
Connecticut valley is seen in Figure 315. Were the beds laid in
their present attitude? What was the nature of the deformation
which they have suffered? When did the intrusion of lava sheets
take place relative to the deformation? What effect have these
sheets on the present topography, and why? Assuming that the
Triassic deformation went on more rapidly than denudation, what
was its effect on the topography of the time? Are there any of its
results remaining in the topography of to-day? Do the Triassic
areas now stand higher or lower than the surrounding country, and
why? How do the Triassic sandstones and shales compare in hardness
with the igneous and metamorphic rocks about them? The Jurassic
strata are wanting over the Triassic areas and over all of eastern
North America. Was this region land or sea, an area of erosion or
sedimentation, during the Jurassic period? In New Jersey,
Pennsylvania, and farther southwest the lowest strata of the next
period, the Cretaceous, rest on the eroded edges of the earlier
rocks. The surface on which they lie is worn so even that we must
believe that at the opening of the Cretaceous the oldland of
Appalachia, including the Triassic areas, had been baseleveled at
least near the coast. When, therefore, did the deformation of the
Triassic rocks occur?

WESTERN NORTH AMERICA. Triassic strata infolded in the Sierra
Nevada Mountains carry marine fossils and reach a thickness of
nearly five thousand feet. California was then under water, and
the site of the Sierra was a subsiding trough slowly filling with
waste from the Great Basin land to the east.

Over a long belt which reaches from Wyoming across Colorado into
New Mexico no Triassic sediments are found, nor is there any
evidence that they were ever present; hence this area was high
land suffering erosion during the Triassic. On each side of it, in
eastern Colorado and about the Black Hills, in western Texas, in
Utah, over the site of the Wasatch Mountains, and southward into
Arizona over the plateaus trenched by the Colorado River, are
large areas of Triassic rocks, sandstones chiefly, with some rock
salt and gypsum. Fossils are very rare and none of them marine.
Here, then, lay broad shallow lakes often salt, and warped basins,
in which the waste of the adjacent uplands gathered. To this
system belong the sandstones of the Garden of the Gods in
Colorado, which later earth movements have upturned with the
uplifted mountain flanks.

The Jurassic was marked with varied oscillations and wide changes
in the outline of sea and land.

Jurassic shales of immense thickness--now metamorphosed into
slates--are found infolded into the Sierra Nevada Mountains. Hence
during Jurassic times the Sierra trough continued to subside, and
enormous deposits of mud were washed into it from the land lying
to the east. Contemporaneous lava flows interbedded with the
strata show that volcanic action accompanied the downwarp, and
that molten rock was driven upward through fissures in the crust
and outspread over the sea floor in sheets of lava.

THE SIERRA DEFORMATION. Ever since the middle of the Silurian, the
Sierra trough had been sinking, though no doubt with halts and
interruptions, until it contained nearly twenty-five thousand feet
of sediment. At the close of the Jurassic it yielded to lateral
pressure and the vast pile of strata was crumpled and upheaved
into towering mountains. The Mesozoic muds were hardened and
squeezed into slates. The rocks were wrenched and broken, and
underground waters began the work of filling their fissures with
gold-bearing quartz, which was yet to wait millions of years
before the arrival of man to mine it. Immense bodies of molten
rock were intruded into the crust as it suffered deformation, and
these appear in the large areas of granite which the later
denudation of the range has brought to light.

The same movements probably uplifted the rocks of the Coast Range
in a chain of islands. The whole western part of the continent was
raised and its seas and lakes were for the most part drained away.

THE BRITISH ISLES. The Triassic strata of the British Isles are
continental, and include breccia beds of cemented talus, deposits
of salt and gypsum, and sandstones whose rounded and polished
grains are those of the wind-blown sands of deserts. In Triassic
times the British Isles were part of a desert extending over much
of northwestern Europe.


The third great system of the Mesozoic includes many formations,
marine and continental, which record a long and complicated
history marked by great oscillations of the crust and wide changes
in the outlines of sea and land.

EARLY CRETACEOUS. In eastern North America the lowest Cretaceous
series comprises fresh-water formations which are traced from
Nantucket across Martha's Vineyard and Long Island, and through
New Jersey southward into Georgia. They rest unconformably on the
Triassic sandstones and the older rocks of the region. The
Atlantic shore line was still farther out than now in the northern
states. Again, as during the Triassic, a warping of the crust
formed a long trough parallel to the coast and to the Appalachian
ridges, but cut off from the sea; and here the continental
deposits of the early Cretaceous were laid.

Along the Gulf of Mexico the same series was deposited under like
conditions over the area known as the Mississippi embayment,
reaching from Georgia northwestward into Tennessee and thence
across into Arkansas and southward into Texas.

In the Southwest the subsidence continued until the transgressing
sea covered most of Mexico and Texas and extended a gulf northward
into Kansas. In its warm and quiet waters limestones accumulated
to a depth of from one thousand to five thousand feet in Texas,
and of more than ten thousand feet in Mexico. Meanwhile the
lowlands, where the Great Plains are now, received continental
deposits; coal swamps stretched from western Montana into British

THE MIDDLE CRETACEOUS. This was a land epoch. The early Cretaceous
sea retired from Texas and Mexico, for its sediments are overlain
unconformably by formations of the Upper Cretaceous. So long was
the time gap between the two series that no species found in the
one occurs in the other.

THE UPPER CRETACEOUS. There now began one of the most remarkable
events in all geological history,--the great Cretaceous
subsidence. Its earlier warpings were recorded in continental
deposits,--wide sheets of sandstone, shale, and some coal,--which
were spread from Texas to British Columbia. These continental
deposits are overlain by a succession of marine formations whose
vast area is shown on the map, Figure 260. We may infer that as
the depression of the continent continued the sea came in far and
wide over the coast lands and the plains worn low during the
previous epochs. Upper Cretaceous formations show that south of
New England the waters of the Atlantic somewhat overlapped the
crystalline rocks of the Piedmont Belt and spread their waste over
the submerged coastal plain. The Gulf of Mexico again covered the
Mississippi embayment, reaching as far north as southern Illinois,
and extended over Texas.

A mediterranean sea now stretched from the Gulf to the arctic
regions and from central Iowa to the eastern shore of the Great
Basin land at about the longitude of Salt Lake City, the Colorado
Mountains rising from it in a chain of islands. Along with minor
oscillations there were laid in the interior sea various
formations of sandstones, shales, and limestones, and from Kansas
to South Dakota beds of white chalk show that the clear, warm
waters swarmed at times with foraminiferal life whose
disintegrating microscopic shells accumulated in this rare

At this epoch a wide sea, interrupted by various islands,
stretched across Eurasia from Wales and western Spain to China,
and spread southward over much of the Sahara. To the west its
waters were clear and on its floor the crumbled remains of
foraminifers gathered in heavy accumulations of calcareous ooze,--
the white chalk of France and England. Sea urchins were also
abundant, and sponges contributed their spicules to form nodules
of flint.

THE LARAMIE. The closing stage of the Cretaceous was marked in
North America by a slow uplift of the land. As the interior sea
gradually withdrew, the warping basins of its floor were filled
with waste from the rising lands about them, and over this wide
area there were spread continental deposits in fresh-water lakes
like the Great Lakes of the present, in brackish estuaries, and in
river plains, while occasional oscillations now and again let in
the sea. There were vast marshes in which there accumulated the
larger part of the valuable coal seams of the West. The Laramie is
the coal-bearing series of the West, as the Pennsylvanian is of
the eastern part of our country.

THE ROCKY MOUNTAIN DEFORMATION. At the close of the Cretaceous we
enter upon an epoch of mountain-making far more extensive than any
which the continent had witnessed. The long belt lying west of the
ancient axes of the Colorado Islands and east of the Great Basin
land had been an area of deposition for many ages, and in its
subsiding troughs Paleozoic and Mesozoic sediments had gathered to
the depth of many thousand feet. And now from Mexico well-nigh to
the Arctic Ocean this belt yielded to lateral pressure. The
Cretaceous limestones of Mexico were folded into lofty mountains.
A massive range was upfolded where the Wasatch Mountains now are,
and various ranges of the Rockies in Colorado and other states
were upridged. However slowly these deformations were effected
they were no doubt accompanied by world-shaking earthquakes, and
it is known that volcanic eruptions took place on a magnificent
scale. Outflows of lava occurred along the Wasatch, the laccoliths
of the Henry Mountains were formed, while the great masses of
igneous rock which constitute the cores of the Spanish Peaks and
other western mountains were thrust up amid the strata. The high
plateaus from which many of these ranges rise had not yet been
uplifted, and the bases of the mountains probably stood near the
level of the sea.

North America was now well-nigh completed. The mediterranean seas
which so often had occupied the heart of the land were done away
with, and the continent stretched unbroken from the foot of the
Sierras on the west to the Fall Line of the Atlantic coastal plain
on the east.

THE MESOZOIC PENEPLAIN. The immense thickness of the Mesozoic
formations conveys to our minds some idea of the vast length of
time involved in the slow progress of its successive ages. The
same lesson is taught as plainly by the amount of denudation which
the lands suffered during the era.

The beginning of the Mesozoic saw a system of lofty mountain
ranges stretching from New York into central Alabama. The end of
this long era found here a wide peneplain crossed by sluggish
wandering rivers and overlooked by detached hills as yet unreduced
to the general level. The Mesozoic era was long enough for the
Appalachian Mountains, upridged at its beginning, to have been
weathered and worn away and carried grain by grain to the sea. The
same plain extended over southern New England. The Taconic range,
uplifted partially at least at the close of the Ordovician, and
the block mountains of the Triassic, together with the pre-
Cambrian mountains of ancient Appalachia, had now all been worn to
a common level with the Allegheny ranges. The Mesozoic peneplain
has been upwarped by later crustal movements and has suffered
profound erosion, but the remnants of it which remain on the
upland of southern New England and the even summits of the
Allegheny ridges suffice to prove that it once existed. The age of
the Mesozoic peneplain is determined from the fact that the lower
Tertiary sediments were deposited on its even surface when at the
close of the era the peneplain was depressed along its edges
beneath the sea.


of lepidodendrons and sigillafids had now vanished from the earth.
The uplands were clothed with conifers, like the Araucarian pines
of South America and Australia. Dense forests of tree ferns throve
in moist regions, and canebrakes of horsetails of modern type, but
with stems reaching four inches in thickness, bordered the lagoons
and marshes. Cycads were exceedingly abundant. These gymnosperms,
related to the pines and spruces in structure and fruiting, but
palmlike in their foliage, and uncoiling their long leaves after
the manner of ferns, culminated in the Jurassic. From the view
point of the botanist the Mesozoic is the Age of Cycads, and after
this era they gradually decline to the small number of species now
existing in tropical latitudes.

PLANT LIFE OF THE CRETACEOUS. In the Lower Cretaceous the
woodlands continued of much the same type as during the Jurassic.
The forerunners now appeared of the modern dicotyls (plants with
two seed leaves), and in the Middle Cretaceous the
monocotyledonous group of palms came in. Palms are so like cycads
that we may regard them as the descendants of some cycad type.

In the UPPER CRETACEOUS, cycads become rare. The highest types of
flowering plants gain a complete ascendency, and forests of modern
aspect cover the continent from the Gulf of Mexico to the Arctic
Ocean. Among the kinds of forest trees whose remains are found in
the continental deposits of the Cretaceous are the magnolia, the
myrtle, the laurel, the fig, the tulip tree, the chestnut, the
oak, beech, elm, poplar, willow, birch, and maple. Forests of
Eucalyptus grew along the coast of New England, and palms on the
Pacific shores of British Columbia. Sequoias of many varieties
ranged far into northern Canada. In northern Greenland there were
luxuriant forests of magnolias, figs, and cycads; and a similar
flora has been disinterred from the Cretaceous rocks of Alaska and
Spitzbergen. Evidently the lands within the Arctic Circle enjoyed
a warm and genial climate, as they had done during the Paleozoic.
Greenland had the temperature of Cuba and southern Florida, and
the time was yet far distant when it was to be wrapped in glacier

INVERTEBRATES. During the long succession of the ages of the
Mesozoic, with their vast geographical changes, there were many
and great changes in organisms. Species were replaced again and
again by others better fitted to the changing environment. During
the Lower Cretaceous alone there were no less than six successive
changes in the faunas which inhabited the limestone-making sea
which then covered Texas. We shall disregard these changes for the
most part in describing the life of the era, and shall confine our
view to some of the most important advances made in the leading

Stromatopora have disappeared. Protozoans and sponges are
exceedingly abundant, and all contribute to the making of Mesozoic
strata. Corals have assumed a more modern type. Sea urchins have
become plentiful; crinoids abound until the Cretaceous, where they
begin their decline to their present humble station.

Trilobites and eurypterids are gone. Ten-footed crustaceans abound
of the primitive long-tailed type (represented by the lobster and
the crayfish), and in the Jurassic there appears the modern short-
tailed type represented by the crabs. The latter type is higher in
organization and now far more common. In its embryological
development it passes through the long-tailed stage; connecting
links in the Mesozoic also indicate that the younger type is the
offshoot of the older.

Insects evolve along diverse lines, giving rise to beetles, ants,
bees, and flies.

Brachiopods have dwindled greatly in the number of their species,
while mollusks have correspondingly increased. The great oyster
family dates from here.

Cephalopods are now to have their day. The archaic Orthoceras
lingers on into the Triassic and becomes extinct, but a remarkable
development is now at hand for the more highly organized
descendants of this ancient line. We have noticed that in the
Devonian the sutures of some of the chambered shells become
angled, evolving the Goniatite type. The sutures now become lobed
and corrugated in Ceratites. The process was carried still
farther, and the sutures were elaborately frilled in the great
order of the Ammonites. It was in the Jurassic that the Ammonites
reached their height. No fossils are more abundant or
characteristic of their age. Great banks of their shells formed
beds of limestone in warm seas the world over.

The ammonite stem branched into a most luxuriant variety of forms.
The typical form was closely coiled like a nautilus. In others the
coil was more or less open, or even erected into a spiral. Some
were hook-shaped, and there were members of the order in which the
shell was straight, and yet retained all the internal structures
of its kind. At the end of the Mesozoic the entire tribe of
ammonites became extinct.

The Belemnite (Greek, belemnon, a dart) is a distinctly higher
type of cephalopod which appeared in the Triassic, became numerous
and varied in the Jurassic and Cretaceous, and died out early in
the Tertiary. Like the squids and cuttlefish, of which it was the
prototype, it had an internal calcareous shell. This consisted of
a chambered and siphuncled cone, whose point was sheathed in a
long solid guard somewhat like a dart. The animal carried an ink
sac, and no doubt used it as that of the modern cuttlefish is
used,--to darken the water and make easy an escape from foes.
Belemnites have sometimes been sketched with fossil sepia, or
india ink, from their own ink sacs. In the belemnites and their
descendants, the squids and cuttlefish, the cephalopods made the
radical change from external to the internal shell. They abandoned
the defensive system of warfare and boldly took up the offensive.
No doubt, like their descendants, the belemnites were exceedingly
active and voracious creatures.

FISHES AND AMPHIBIANS. In the Triassic and Jurassic, little
progress was made among the fishes, and the ganoid was still the
leading type. In the Cretaceous the teleosts, or bony fishes, made
their appearance, while ganoids declined toward their present
subordinate place.

The amphibians culminated in the Triassic, some being formidable
creatures as large as alligators. They were still of the primitive
Paleozoic types. Their pygmy descendants of more modern types are
not found until later, salamanders appearing first in the
Cretaceous, and frogs at the beginning of the Cenozoic.

No remains of amphibians have been discovered in the Jurassic. Do
you infer from this that there were none in existence at that


The great order of Reptiles made its advent in the Permian,
culminated in the Triassic and Jurassic, and began to decline in
the Cretaceous. The advance from the amphibian to the reptile was
a long forward step in the evolution of the vertebrates. In the
reptile the vertebrate skeleton now became completely ossified.
Gills were abandoned and breathing was by lungs alone. The
development of the individual from the egg to maturity was
uninterrupted by any metamorphosis, such as that of the frog when
it passes from the tadpole stage. Yet in advancing from the
amphibian to the reptile the evolution of the vertebrate was far
from finished. The cold-blooded, clumsy and sluggish, small-
brained and unintelligent reptile is as far inferior to the higher
mammals, whose day was still to come, as it is superior to the
amphibian and the fish.

The reptiles of the Permian, the earliest known, were much like
lizards in form of body. Constituting a transition type between
the amphibians on the one hand, and both the higher reptiles and
the mammals on the other, they retained the archaic biconcave
vertebra of the fish and in some cases the persistent notochord,
while some of them, the theromorphs, possessed characters allying
them with mammals. In these the skull was remarkably similar to
that of the carnivores, or flesh-eating mammals, and the teeth,
unlike the teeth of any later reptiles, were divisible into
incisors, canines, and molars, as are the teeth of mammals.

At the opening of the Mesozoic era reptiles were the most highly
organized and powerful of any animals on the earth. New ranges of
continental extent were opened to them, food was abundant, the
climate was congenial, and they now branched into very many
diverse types which occupied and ruled all fields,--the land, the
air, and the sea. The Mesozoic was the Age of Reptiles.

the evolution of the few reptilian types which have survived to
the present.

Crocodiles, the highest of existing reptiles, are a very ancient
order, dating back to the lower Jurassic, and traceable to earlier
ancestral, generalized forms, from which sprang several other
orders also.

Turtles and tortoises are not found until the early Jurassic, when
they already possessed the peculiar characteristics which set them
off so sharply from other reptiles. They seem to have lived at
first in shallow water and in swamps, and it is not until after
the end of the Mesozoic that some of the order became adapted to
life on the land.

The largest of all known turtles, Archelon, whose home was the
great interior Cretaceous sea, was fully a dozen feet in length
and must have weighed at least two tons. The skull alone is a yard

Lizards and snakes do not appear until after the close of the
Mesozoic, although their ancestral lines may be followed back into
the Cretaceous.

We will now describe some of the highly specialized orders
peculiar to the Mesozoic.

LAND REPTILES. The DINOSAURS (terrible reptiles) are an extremely
varied order which were masters of the land from the late Trias
until the close of the Mesozoic era. Some were far larger than
elephants, some were as small as cats; some walked on all fours,
some were bipedal; some fed on the luxuriant tropical foliage, and
others on the flesh of weaker reptiles. They may be classed in
DINOSAURS, and the BEAKED DINOSAURS,--the latter two divisions
being herbivorous.

The FLESH-EATING DINOSAURS are the oldest known division of the
order, and their characteristics are shown in Figure 329. As a
class, reptiles are egg layers (oviparous); but some of the flesh-
eating dinosaurs are known to have been VIVIPAROUS, i.e. to have
brought forth their young alive. This group was the longest-lived
of any of the three, beginning in the Trias and continuing to the
close of the Mesozoic era.

Contrast the small fore limbs, used only for grasping, with the
powerful hind limbs on which the animal stalked about. Some of the
species of this group seem to have been able to progress by
leaping in kangaroo fashion. Notice the sharp claws, the ponderous
tail, and the skull set at right angles with the spinal column.
The limb bones are hollow. The ceratosaurs reached a length of
some fifteen feet, and were not uncommon in Colorado and the
western lands in Jurassic times.

The REPTILE-FOOTED DINOSAURS (Sauropoda) include some of the
biggest brutes which ever trod the ground. One of the largest,
whose remains are found entombed in the Jurassic rocks of Wyoming
and Colorado, is shown in Figure 330.

Note the five digits on the hind feet, the quadrupedal gait, the
enormous stretch of neck and tail, the small head aligned with the
vertebral column. Diplodocus was fully sixty-five feet long and
must have weighed about twenty tons. The thigh bones of the
Sauropoda are the largest bones which ever grew. That of a genus
allied to the Diplodocus measures six feet and eight inches, and
the total length of the animal must have been not far from eighty
feet, the largest land animal known.

The Sauropoda became extinct when their haunts along the rivers
and lakes of the western plains of Jurassic times were invaded by
the Cretaceous interior sea.

The BEAKED DINOSAURS(Predentata) were distinguished by a beak
sheathed with horn carried in front of the tooth-set jaw, and
used, we may imagine, in stripping the leaves and twigs of trees
and shrubs. We may notice only two of the most interesting types.

STEGOSAURUS (plated reptile) takes its name from the double row of
bony plates arranged along its back. The powerful tail was armed
with long spines, and the thick skin was defended with irregular
bits of bone implanted in it. The brain of the stegosaur was
smaller than that of any land vertebrate, while in the sacrum the
nerve canal was enlarged to ten times the capacity of the brain
cavity of the skull. Despite their feeble wits, this well-armored
family lived on through millions of years which intervened between
their appearance, at the opening of the Jurassic, and the close of
the Cretaceous, when they became extinct.

A less stupid brute than the stegosaur was TRICERATOPS, the
dinosaur of the three horns,--one horn carried on the nose, and a
massive pair set over the eyes. Note the enormous wedge-shaped
skull, with its sharp beak, and the hood behind resembling a
fireman's helmet. Triceratops was fully twenty-five feet long, and
of twice the bulk of an elephant. The family appeared in the Upper
Cretaceous and became extinct at its close. Their bones are found
buried in the fresh-water deposits of the time from Colorado to
Montana and eastward to the Dakotas.

MARINE REPTILES. In the ocean, reptiles occupied the place now
held by the aquatic mammals, such as whales and dolphins, and
their form and structure were similarly modified to suit their
environment. In the Ichthyosaurus (fish reptile), for example, the
body was fishlike in form, with short neck and large, pointed head
(Fig. 333).

A powerful tail, whose flukes were set vertical, and the lower one
of which was vertebrated, served as propeller, while a large
dorsal fin was developed as a cutwater. The primitive biconcave
vertebrae of the fish and of the early land vertebrates were
retained, and the limbs degenerated into short paddles. The skin
of the ichthyosaur was smooth like that of a whale, and its food
was largely fish and cephalopods, as the fossil contents of its
stomach prove.

These sea monsters disported along the Pacific shore over northern
California in Triassic times, and the bones of immense members of
the family occur in the Jurassic strata of Wyoming. Like whales
and seals, the ichthyosaurs were descended from land vertebrates
which had become adapted to a marine habitat.

PLESIOSAURS were another order which ranged throughout the
Mesozoic. Descended from small amphibious animals, they later
included great marine reptiles, characterized in the typical genus
by long neck, snakelike head, and immense paddles. They swam in
the Cretaceous interior sea of western North America.

MOSASAURS belong to the same order as do snakes and lizards, and
are an offshoot of the same ancestral line of land reptiles. These
snakelike creatures--which measured as much as forty-five feet in
length--abounded in the Cretaceous seas. They had large conical
teeth, and their limbs had become stout paddles.

The lower jaw of the mosasaur was jointed; the quadrate bone,
which in all reptiles connects the bone of the lower jaw with the
skull, was movable, and as in snakes the lower jaw could be used
in thrusting prey down the throat. The family became extinct at
the end of the Mesozoic, and left no descendants. One may imitate
the movement of the lower jaw of the mosasaur by extending the
arms, clasping the hands, and bending the elbows.

FLYING REPTILES. The atmosphere, which had hitherto been tenanted
only by insects, was first conquered by the vertebrates in the
Mesozoic. Pterosaurs, winged reptiles, whose whole organism was
adapted for flight through the air, appeared in the Jurassic and
passed off the stage of existence before the end of the
Cretaceous. The bones were hollow, as are those of birds. The
sternum, or breastbone, was given a keel for the attachment of the
wing muscles. The fifth finger, prodigiously lengthened, was
turned backward to support a membrane which was attached to the
body and extended to the base of the tail. The other fingers were
free, and armed with sharp and delicate claws, as shown in Figures
336 and 337.

These "dragons of the air" varied greatly in size; some were as
small as sparrows, while others surpassed in stretch of wing the
largest birds of the present day. They may be divided into two
groups. The earliest group comprises genera with jaws set with
teeth, and with long tails sometimes provided with a rudderlike
expansion at the end. In their successors of the later group the
tail had become short, and in some of the genera the teeth had
disappeared. Among the latest of the flying reptiles was
ORNITHOSTOMA (bird beak), the largest creature which ever flew,
and whose remains are imbedded in the offshore deposits of the
Cretaceous sea which held sway over our western plains.
Ornithostoma's spread of wings was twenty feet. Its bones were a
marvel of lightness, the entire skeleton, even in its petrified
condition, not weighing more than five or six pounds. The sharp
beak, a yard long, was toothless and bird-like, as its name

BIRDS. The earliest known birds are found in the Jurassic, and
during the remainder of the Mesozoic they contended with the
flying reptiles for the empire of the air. The first feathered
creatures were very different from the birds of to-day. Their
characteristics prove them an offshoot of the dinosaur line of
reptiles. ARCHAEOPTERYX (ANCIENT BIRD) (Fig. 338) exhibits a
strange mingling of bird and reptile. Like birds, it was fledged
with perfect feathers, at least on wings and tail, but it retained
the teeth of the reptile, and its long tail was vertebrated, a
pair of feathers springing from each joint. Throughout the
Jurassic and Cretaceous the remains of birds are far less common
than those of flying reptiles, and strata representing hundreds of
thousands of years intervene between Archaeopteryx and the next
birds of which we know, whose skeletons occur in the Cretaceous
beds of western Kansas.

MAMMALS. So far as the entries upon the geological record show,
mammals made their advent in a very humble way during the Trias.
These earliest of vertebrates which suckle their young were no
bigger than young kittens, and their strong affinities with the
theromorphs suggest that their ancestors are to be found among
some generalized types of that order of reptiles.

During the long ages of the Mesozoic, mammals continued small and
few, and were completely dominated by the reptiles. Their remains
are exceedingly rare, and consist of minute scattered teeth,--with
an occasional detached jaw,--which prove them to have been flesh
or insect eaters. In the same way their affinities are seen to be
with the lowest of mammals,--the MONOTREMES and MARSUPIALS. The
monotremes,--such as the duckbill mole and the spiny ant-eater of
Australia, reproduce by means of eggs resembling those of
reptiles; the marsupials, such as the opossum and the kangaroo,
bring forth their young alive, but in a very immature condition,
and carry them for some time after birth in the marsupium, a pouch
on the ventral side of the body.



THE CENOZOIC ERA. The last stages of the Cretaceous are marked by
a decadence of the reptiles. By the end of that period the
reptilian forms characteristic of the time had become extinct one
after another, leaving to represent the class only the types of
reptiles which continue to modern times. The day of the ammonite
and the belemnite also now drew to a close, and only a few of
these cephalopods were left to survive the period. It is therefore
at the close of the Cretaceous that the line is drawn which marks
the end of the Middle Age of geology and the beginning of the
Cenozoic era, the era of modern life,--the Age of Mammals.

In place of the giant reptiles, mammals now become masters of the
land, appearing first in generalized types which, during the long
ages of the era, gradually evolve to higher forms, more
specialized and ever more closely resembling the mammals of the
present. In the atmosphere the flying dragons of the Mesozoic give
place to birds and bats. In the sea, whales, sharks, and teleost
fishes of modern types rule in the stead of huge swimming
reptiles. The lower vertebrates, the invertebrates of land and
sea, and the plants of field and forest take on a modern aspect,
and differ little more from those of to-day than the plants and
animals of different countries now differ from one another. From
the beginning of the Cenozoic era until now there is a steadily
increasing number of species of animals and plants which have
continued to exist to the present time.

The Cenozoic era comprises two divisions,--the TERTIARY period and
the QUATERNARY period.

In the early days of geology the formations of the entire
geological record, so far as it was then known, were divided into
three groups,--the PRIMARY, the SECONDARY (now known as the
Mesozoic), and the TERTIARY, When the third group was subdivided
into two systems, the term Tertiary was retained for the first
system of the two, while the term QUATERNARY was used to designate
the second.

DIVISIONS OF THE TERTIARY. The formations of the Tertiary are
grouped in three divisions,--the PLIOCENE (more recent), the
MIOCENE (less recent), and the EOCENE (the dawn of the recent).
Each of these epochs is long and complex. Their various sub-
divisions are distinguished each by its own peculiar organisms,
and the changes of physical geography recorded in their strata. In
the rapid view which we are compelled to take we can note only a
few of the most conspicuous events of the period.

Tertiary rocks of eastern North America are marine deposits and
occupy the coastal lowlands of the Atlantic and Gulf states (Fig.
260). In New England, Tertiary beds occur on the island of
Martha's Vineyard, but not on the mainland; hence the shore line
here stood somewhat farther out than now. From New Jersey
southward the earliest Tertiary sands and clays, still
unconsolidated, leave only a narrow strip of the edge of the
Cretaceous between them and the Triassic and crystalline rocks of
the Piedmont oldland; hence the Atlantic shore here stood farther
in than now, and at the beginning of the period the present
coastal plain was continental delta. A broad belt of Tertiary sea-
laid limestones, sandstones, and shales surrounds the Gulf of
Mexico and extends northward up the Mississippi embayment to the
mouth of the Ohio River; hence the Gulf was then larger than at
present, and its waters reached in a broad bay far up the
Mississippi valley.

Along the Atlantic coast the Mesozoic peneplain may be traced
shoreward to where it disappears from view beneath an
unconformable cover of early Tertiary marine strata. The beginning
of the Tertiary was therefore marked by a subsidence. The wide
erosion surface which at the close of the Mesozoic lay near sea
level where the Appalachian Mountains and their neighboring
plateaus and uplands now stand was lowered gently along its
seaward edge beneath the Tertiary Atlantic to receive a cover of
its sediments.

As the period progressed slight oscillations occurred from time to
time. Strips of coastal plain were added to the land, and as early
as the close of the Miocene the shore lines of the Atlantic and
Gulf states had reached well-nigh their present place. Louisiana
and Florida were the last areas to emerge wholly from the sea,--
Florida being formed by a broad transverse upwarp of the
continental delta at the opening of the Miocene, forming first an
island, which afterwards was joined to the mainland.

THE PACIFIC COAST. Tertiary deposits with marine fossils occur
along the western foothills of the Sierra Nevadas, and are
crumpled among the mountain masses of the Coast Ranges; it is
hence inferred that the Great Valley of California was then a
border sea, separated from the ocean by a chain of mountainous
islands which were upridged into the Coast Ranges at a still later
time. Tertiary marine strata are spread over the lower Columbia
valley and that of Puget Sound, showing that the Pacific came in
broadly there.

the Mesozoic were marked, as we have seen, by the upheaval of the
Rocky Mountains and other western ranges. The bases of the
mountains are now skirted by widespread Tertiary deposits, which
form the highest strata of the lofty plateaus from the level of
whose summits the mountains rise. Like the recent alluvium of the
Great Valley of California, these deposits imply low-lying lands
when they were laid, and therefore at that time the mountains rose
from near sea level. But the height at which the Tertiary strata
now stand--five thousand feet above the sea at Denver, and twice
that height in the plateaus of southern Utah--proves that the
plateaus of which the Tertiary strata form a part have been
uplifted during the Cenozoic. During their uplift, warping formed
extensive basins both east and west of the Rockies, and in these
basins stream-swept and lake-laid waste gathered to depths of
hundreds and thousands of feet, as it is accumulating at present
in the Great Valley of California and on the river plains of
Turkestan. The Tertiary river deposits of the High Plains have
already been described. How widespread are these ancient river
plains and beds of fresh-water lakes may be seen in the map of
Figure 260.

THE BAD LANDS. In several of the western states large areas of
Tertiary fresh-water deposits have been dissected to a maze of
hills whose steep sides are cut with innumerable ravines. The
deposits of these ancient river plains and lake beds are little
cemented and because of the dryness of the climate are unprotected
by vegetation; hence they are easily carved by the wet-weather
rills of scanty and infrequent rains. These waterless, rugged
surfaces were named by the early French explorers the BAD LANDS
because they were found so difficult to traverse. The strata of
the Bad Lands contain vast numbers of the remains of the animals
of Tertiary times, and the large amount of barren surface exposed
to view makes search for fossils easy and fruitful. These desolate
tracts are therefore frequently visited by scientific collecting

MOUNTAIN MAKING IN THE TERTIARY. The Tertiary period included
epochs when the earth's crust was singularly unquiet. From time to
time on all the continents subterranean forces gathered head, and
the crust was bent and broken and upridged in lofty mountains.

The Sierra Nevada range was formed, as we have seen, by strata
crumpling at the end of the Jurassic. But since that remote time
the upfolded mountains had been worn to plains and hilly uplands,
the remnants of whose uplifted erosion surfaces may now be traced
along the western mountain slopes. Beginning late in the Tertiary,
the region was again affected by mountain-making movements. A
series of displacements along a profound fault on the eastern side
tilted the enormous earth block of the Sierras to the west,
lifting its eastern edge to form the lofty crest and giving to the
range a steep eastern front and a gentle descent toward the

The Coast Ranges also have had a complex history with many
vicissitudes. The earliest foldings of their strata belong to the
close of the Jurassic, but it was not until the end of the Miocene
that the line of mountainous islands and the heavy sediments which
had been deposited on their submerged flanks were crushed into a
continuous mountain chain. Thick Pliocene beds upon their sides
prove that they were depressed to near sea level during the later
Tertiary. At the close of the Pliocene the Coast Ranges rose along
with the upheaval of the Sierra, and their gradual uplift has
continued to the present time.

The numerous north-south ranges of the Great Basin and the Mount
Saint Elias range of Alaska were also uptilted during the

During the Tertiary period many of the loftiest mountains of the
earth--the Alps, the Apennines, the Pyrenees, the Atlas, the
Caucasus, and the Himalayas--received the uplift to which they owe
most of their colossal bulk and height, as portions of the
Tertiary sea beds now found high upon their flanks attest. In the
Himalayas, Tertiary marine limestones occur sixteen thousand five
hundred feet above sea level.

VOLCANIC ACTIVITY IN THE TERTIARY. The vast deformations of the
Tertiary were accompanied on a corresponding scale by outpourings
of lava, the outburst of volcanoes, and the intrusion of molten
masses within the crust. In the Sierra Nevadas the Miocene river
gravels of the valleys of the western slope, with their placer
deposits of gold, were buried beneath streams of lava and beds of
tuff. Volcanoes broke forth along the Rocky Mountains and on the
plateaus of Utah, New Mexico, and Arizona.

Mount Shasta and the immense volcanic piles of the Cascades date
from this period. The mountain basin of the Yellowstone Park was
filled to a depth of several thousand feet with tuffs and lavas,
the oldest dating as far back as the beginning of the Tertiary.
Crandall volcano was reared in the Miocene and the latest
eruptions of the Park are far more recent.

THE COLUMBIA AND SNAKE RIVER LAVAS. Still more important is the
plateau of lava, more than two hundred thousand square miles in
area, extending from the Yellowstone Park to the Cascade
Mountains, which has been built from Miocene times to the present.

Over this plateau, which occupies large portions of Idaho,
Washington, and Oregon, and extends into northern California and
Nevada, the country rock is basaltic lava. For thousands of square
miles the surface is a lava plain which meets the boundary
mountains as a lake or sea meets a rugged and deeply indented
coast. The floods of molten rock spread up the mountain valleys
for a score of miles and more, the intervening spurs rising above
the lava like long peninsulas, while here and there an isolated
peak was left to tower above the inundation like an island off a
submerged shore.

The rivers which drain the plateau--the Snake, the Columbia, and
their tributaries--have deeply trenched it, yet their canyons,
which reach the depth of several thousand feet, have not been worn
to the base of the lava except near the margin and where they cut
the summits of mountains drowned beneath the flood. Here and there
the plateau has been deformed. It has been upbent into great
folds, and broken into immense blocks of bedded lava, forming
mountain ranges, which run parallel with the Pacific coast line.
On the edges of these tilted blocks the thickness of the lava is
seen to be fully five thousand feet. The plateau has been built,
like that of Iceland, of innumerable overlapping sheets of lava.
On the canyon walls they weather back in horizontal terraces and
long talus slopes. One may distinguish each successive flow by its
dense central portion, often jointed with large vertical columns,
and the upper portion with its mass of confused irregular columns
and scoriaceous surface. The average thickness of the flows seems
to be about seventy-five feet.

The plateau was long in building. Between the layers are found in
places old soil beds and forest grounds and the sediments of
lakes. Hence the interval between the flows in any locality was
sometimes long enough for clays to gather in the lakes which
filled depressions in the surface. Again and again the surface of
the black basalt was reddened by oxidation and decayed to soil,
and forests had time to grow upon it before the succeeding
inundation sealed the sediments and soils away beneath a sheet of
stone. Near the edges of the lava plain, rivers from the
surrounding mountains spread sheets of sand and gravel on the
surface of one flow after another. These pervious sands,
interbedded with the lava, become the aquifers of artesian wells.

In places the lavas rest on extensive lake deposits, one thousand
feet deep, and Miocene in age as their fossils prove. It is to the
middle Tertiary, then, that the earliest flows and the largest
bulk of the great inundation belong. So ancient are the latest
floods in the Columbia basin that they have weathered to a
residual yellow clay from thirty to sixty feet in depth and
marvelously rich in the mineral substances on which plants feed.

In the Snake River valley the latest lavas are much younger. Their
surfaces are so fresh and undecayed that here the effusive
eruptions may well have continued to within the period of human
history. Low lava domes like those of Iceland mark where last the
basalt outwelled and spread far and wide before it chilled (Fig.
341). In places small mounds of scoria show that the eruptions
were accompanied to a slight degree by explosions of steam. So
fluid was this superheated lava that recent flows have been traced
for more than fifty miles.

The rocks underlying the Columbia lavas, where exposed to view,
are seen to be cut by numerous great dikes of dense basalt, which
mark the fissures through which the molten rock rose to the

The Tertiary included times of widespread and intense volcanic
action in other continents as well as in North America. In Europe,
Vesuvius and Etna began their career as submarine volcanoes in
connection with earth movements which finally lifted Pliocene
deposits in Sicily to their present height,--four thousand feet
above the sea. Volcanoes broke forth in central France and
southern Germany, in Hungary and the Carpathians. Innumerable
fissures opened in the crust from the north of Ireland and the
western islands of Scotland to the Faroes, Iceland, and even to
arctic Greenland; and here great plateaus were built of flows of
basalt similar to that of the Columbia River. In India, at the
opening of the Tertiary, there had been an outwelling of basalt,
flooding to a depth of thousands of feet two hundred thousand
square miles of the northwestern part of the peninsula, and
similar inundations of lava occurred where are now the table-lands
of Abyssinia. From the middle Tertiary on, Asia Minor, Arabia, and
Persia were the scenes of volcanic action. In Palestine the rise
of the uplands of Judea at the close of the Eocene, and the
downfaulting of the Jordan valley were followed by volcanic
outbursts. In comparison with the middle Tertiary, the present is
a time of volcanic inactivity and repose.

plateaus built at various times during the Tertiary and at its
commencement have been profoundly carved by erosive agents. The
Sierra Nevada Mountains have been dissected on the western slope
by such canyons as those of King's River and the Yosemite. Six
miles of strata have been denuded from parts of the Wasatch
Mountains since their rise at the beginning of the era. From the
Colorado plateaus, whose uplift dates from the same time, there
have been stripped off ten thousand feet of strata over thousands
of square miles, and the colossal canyon of the Colorado has been
cut after this great denudation had been mostly accomplished.

On the eastern side of the continent, as we have seen, a broad
peneplain had been developed by the close of the Cretaceous. The
remnants of this old erosion surface are now found upwarped to
various heights in different portions of its area. In southern New
England it now stands fifteen hundred feet above the sea in
western Massachusetts, declining thence southward and eastward to
sea level at the coast. In southwestern Virginia it has been
lifted to four thousand feet above the sea. Manifestly this upwarp
occurred since the peneplain was formed; it is later than the
Mesozoic, and the vast dissection which the peneplain has suffered
since its uplift must belong to the successive cycles of Cenozoic

Revived by the uplift, the streams of the area trenched it as
deeply as its elevation permitted, and reaching grade, opened up
wide valleys and new peneplains in the softer rocks. The
Connecticut valley is Tertiary in age, and in the weak Triassic
sandstones has been widened in places to fifteen miles. Dating
from the same time are the valleys of the Hudson, the Susquehanna,
the Delaware, the Potomac, and the Shenandoah.

In Pennsylvania and the states lying to the south the Mesozoic
peneplain lies along the summits of the mountain ridges. On the
surface of this ancient plain, Tertiary erosion etched out the
beautifully regular pattern of the Allegheny mountain ridges and
their intervening valleys. The weaker strata of the long, regular
folds were eroded into longitudinal valleys, while the hard
Paleozoic sandstones, such as the Medina and the Pocono, were left
in relief as bold mountain walls whose even crests rise to the
common level of the ancient plain. From Virginia far into Alabama
the great Appalachian valley was opened to a width in places of
fifty miles and more, along a belt of intensely folded and faulted
strata where once was the heart of the Appalachian Mountains. In
Figure 70 the summit of the Cumberland plateau (ab) marks the
level of the Mesozoic peneplain, while the lower erosion levels
are Tertiary and Quaternary in age.


VEGETATION AND CLIMATE. The highest plants in structure, the
DICOTYLS (such as our deciduous forest trees) and the MONOCOTYLS
(represented by the palms), were introduced during the Cretaceous.
The vegetable kingdom reached its culmination before the animal
kingdom, and if the dividing line between the Mesozoic and the
Cenozoic were drawn according to the progress of plant life, the
Cretaceous instead of the Tertiary would be made the opening
period of the modern era.

The plants of the Tertiary belonged, for the most part, to genera
now living; but their distribution was very different from that of
the flora of to-day. In the earlier Tertiary, palms flourished
over northern Europe, and in the northwestern United States grew
the magnolia and laurel, along with the walnut, oak, and elm. Even
in northern Greenland and in Spitzbergen there were lakes covered
with water lilies and surrounded by forests of maples, poplars,
limes, the cypress of our southern states, and noble sequoias
similar to the "big trees" and redwoods of California. A warm
climate like that of the Mesozoic, therefore, prevailed over North
America and Europe, extending far toward the pole. In the later
Tertiary the climate gradually became cooler. Palms disappeared
from Europe, and everywhere the aspect of forests and open lands
became more like that of to-day. Grasses became abundant,
furnishing a new food for herbivorous animals.

ANIMAL LIFE OF THE TERTIARY. Little needs to be said of the
Tertiary invertebrates, so nearly were they like the invertebrates
of the present. Even in the Eocene, about five per cent of marine
shells were of species still living, and in the Pliocene the
proportion had risen to more than one half.

Fishes were of modern types. Teleosts were now abundant. The ocean
teemed with sharks, some of them being voracious monsters seventy-
five feet and even more in length, with a gape of jaw of six feet,
as estimated by the size of their enormous sharp-edged teeth.

Snakes are found for the first time in the early Tertiary. These
limbless reptiles, evolved by degeneration from lizardlike
ancestors, appeared in nonpoisonous types scarcely to be
distinguished from those of the present day.

MAMMALS OF THE EARLY TERTIARY. The fossils of continental deposits
of the earliest Eocene show that a marked advance had now been
made in the evolution of the Mammalia. The higher mammals had
appeared, and henceforth the lower mammals--the monotremes and
the marsupials--are reduced to a subordinate place.

These first true mammals were archaic and generalized in
structure. Their feet were of the primitive type, with five toes
of about equal length. They were also PLANTIGRADES,--that is, they
touched the ground with the sole of the entire foot from toe to
heel. No foot had yet become adapted to swift running by a
decrease in the number of digits and by lifting the heel and sole
so that only the toes touch the ground,--a tread called
DIGITIGRADE. Nor was there yet any foot like that of the cats,
with sharp retractile claws adapted to seizing and tearing the
prey. The forearm and the lower leg each had still two separate
bones (ulna and radius, fibula and tibia), neither pair having
been replaced with a single strong bone, as in the leg of the
horse. The teeth also were primitive in type and of full number.
The complex heavy grinders of the horse and elephant, the sharp
cutting teeth of the carnivores, and the cropping teeth of the
grass eaters were all still to come.

Phenacodus is a characteristic genus of the early Eocene, whose
species varied in size from that of a bulldog to that of an animal
a little larger than a sheep. Its feet were primitive, and their
five toes bore nails intermediate in form between a claw and a
hoof. The archaic type of teeth indicates that the animal was
omnivorous in diet. A cast of the brain cavity shows that, like
its associates of the time, its brain was extremely small and
nearly smooth, having little more than traces of convolutions.

The long ages of the Eocene and the following epochs of the
Tertiary were times of comparatively rapid evolution among the
Mammalia. The earliest forms evolved along diverging lines toward
the various specialized types of hoofed mammals, rodents,
carnivores, proboscidians, the primates, and the other mammalian
orders as we know them now. We must describe the Tertiary mammals
very briefly, tracing the lines of descent of only a few of the
more familiar mammals of the present.

THE HORSE. The pedigree of the horse runs back into the early
Eocene through many genera and species to a five-toed, [Footnote:
Or, more accurately, with four perfect toes and a rudimentary
fifth corresponding to the thumb.] short-legged ancestor little
bigger than a cat. Its descendants gradually increased in stature
and became better and better adapted to swift running to escape
their foes. The leg became longer, and only the tip of the toes
struck the ground. The middle toe (digit number three), originally
the longest of the five, steadily enlarged, while the remaining
digits dwindled and disappeared. The inner digit, corresponding to
the great toe and thumb, was the first to go. Next number five,
the little finger, was also dropped. By the end of the Eocene a
three-toed genus of the horse family had appeared, as large as a
sheep. The hoof of digit number three now supported most of the
weight, but the slender hoofs of digits two and four were still
serviceable. In the Miocene the stature of the ancestors of the
horse increased to that of a pony. The feet were still three-toed,
but the side hoofs were now mere dewclaws and scarcely touched the
ground. The evolution of the family was completed in the Pliocene.

The middle toe was enlarged still more, the side toes were
dropped, and the palm and foot bones which supported them were
reduced to splints.

While these changes were in progress the radius and ulna of the
fore limb became consolidated to a single bone; and in the hind
limb the fibula dwindled to a splint, while the tibia was
correspondingly enlarged. The molars, also gradually lengthened,
and became more and more complex on their grinding surface; the
neck became longer; the brain steadily increased in size and its
convolutions became more abundant. The evolution of the horse has
made for greater fleetness and intelligence.

THE RHINOCEROS AND TAPIR. These animals, which are grouped with
the horse among the ODD-TOED (perissodactyl) mammals, are now
verging toward extinction. In the rhinoceros, evolution seems to
have taken the opposite course from that of the horse. As the
animal increased in size it became more clumsy, its limbs became
shorter and more massive, and, perhaps because of its great
weight, the number of digits were not reduced below the number
three. Like other large herbivores, the rhinoceros, too slow to
escape its enemies by flight, learned to withstand them. It
developed as its means of defense a nasal horn.

Peculiar offshoots of the line appeared at various times in the
Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks,
resembling in aspect the hippopotamus, lived along the water
courses of the plains east of the Rockies, and its bones are now
found by the thousands in the Miocene of Kansas. Another developed
along a line parallel to that of the horse, and herds of these
light-limbed and swift-footed running rhinoceroses ranged the
Great Plains from the Dakotas southward.

The tapirs are an ancient family which has changed but little
since it separated from the other perissodactyl stocks in the
early Tertiary. At present, tapirs are found only in South America
and southern Asia,--a remarkable distribution which we could not
explain were it not that the geological record shows that during
Tertiary times tapirs ranged throughout the northern hemisphere,
making their way to South America late in that period. During the
Pleistocene they became extinct over all the intervening lands
between the widely separated regions where now they live. The
geographic distribution of animals, as well as their relationships
and origins, can be understood only through a study of their
geological history.

THE PROBOSCIDIANS. This unique order of hoofed mammals, of which
the elephant is the sole survivor, began, so far as known, in the
Eocene, in Egypt, with a piglike ancestor the size of a small
horse, with cheek teeth like the Mastodon's, but wanting both
trunk and tusks. A proboscidian came next with four short tusks,
and in the Miocene there followed a Mastodon (Fig. 346) armed with
two pairs of long, straight tusks on which rested a flexible

The DINOTHERE was a curious offshoot of the line, which developed
in the Miocene in Europe. In this immense proboscidian, whose
skull was three feet long, the upper pair of tusks had
disappeared, and those of the lower jaw were bent down with a
backward curve in walrus fashion.

In the true ELEPHANTS, which do not appear until near the close of
the Tertiary, the lower jaw loses its tusks and the grinding teeth
become exceedingly complex in structure. The grinding teeth of the
mastodon had long roots and low crowns crossed by four or five
peaked enameled ridges. In the teeth of the true elephants the
crown has become deep, and the ridges of enamel have changed to
numerous upright, platelike folds, their interspaces filled with
cement. The two genera--Mastodon and Elephant--are connected by
species whose teeth are intermediate in pattern. The proboscidians
culminated in the Pliocene, when some of the giant elephants
reached a height of fourteen feet.

THE ARTIODACTYLS comprise the hoofed Mammalia which have an even
number of toes, such as cattle, sheep, and swine. Like the
perissodactyls, they are descended from the primitive five-toed
plantigrade mammals of the lowest Eocene. In their evolution,
digit number one was first dropped, and the middle pair became
larger and more massive, while the side digits, numbers two and
five, became shorter, weaker, and less serviceable. The FOUR-TOED
ARTIODACTYLS culminated in the Tertiary; at present they are
represented only by the hippopotamus and the hog. Along the main
line of the evolution of the artiodactyls the side toes, digits
two and five, disappeared, leaving as proof that they once existed
the corresponding bones of palm and sole as splints. The TWO-TOED
ARTIODACTYLS, such as the camels, deer, cattle, and sheep, are now
the leading types of the herbivores.

SWINE AND PECCARIES are two branches of a common stock, the first
developing in the Old World and the second in the New. In the
Miocene a noticeable offshoot of the line was a gigantic piglike
brute, a root eater, with a skull a yard in length, whose remains
are now found in Colorado and South Dakota.

CAMELS AND LLAMAS. The line of camels and llamas developed in
North America, where the successive changes from an early Eocene
ancestor, no larger than a rabbit, are traced step by step to the
present forms, as clearly as is the evolution of the horse. In the
late Miocene some of the ancestral forms migrated to the Old World
by way of a land connection where Bering Strait now is, and there
gave rise to the camels and dromedaries. Others migrated into
South America, which had now been connected with our own
continent, and these developed into the llamas and guanacos, while
those of the race which remained in North America became extinct
during the Pleistocene.

Some peculiar branches of the camel stem appeared in North
America. In the Pliocene arose a llama with the long neck and
limbs of a giraffe, whose food was cropped from the leaves and
branches of trees. Far more generalized in structure was the
Oreodon, an animal related to the camels, but with distinct
affinities also with other lines, such as those of the hog and
deer. These curious creatures were much like the peccary in
appearance, except for their long tails. In the middle Eocene they
roamed in vast herds from Oregon to Kansas and Nebraska.

THE RUMINANTS. This division of the artiodactyls includes
antelopes, deer, oxen, bison, sheep, and goats,--all of which
belong to a common stock which took its rise in Europe in the
upper Eocene from ancestral forms akin to those of the camels. In
the Miocene the evolution of the two-toed artiodactyl foot was
well-nigh completed. Bonelike growths appeared on the head, and
the two groups of the ruminants became specialized,--the deer with
bony antlers, shed and renewed each year, and the ruminants with
hollow horns, whose two bony knobs upon the skull are covered with
permanent, pointed, horny sheaths.

The ruminants evolved in the Old World, and it was not until the
later Miocene that the ancestors of the antelope and of some deer
found their way to North America. Mountain sheep and goats, the
bison and most of the deer, did not arrive until after the close
of the Tertiary, and sheep and oxen were introduced by man.

The hoofed mammals of the Tertiary included many offshoots from
the main lines which we have traced. Among them were a number of
genera of clumsy, ponderous brutes, some almost elephantine in
their bulk.

THE CARNIVORES. The ancestral lines of the families of the flesh
eaters--such as the cats (lions, tigers, etc.), the bears, the
hyenas, and the dogs (including wolves and foxes)--converge in the
creodonts of the early Eocene,--an order so generalized that it
had affinities not only with the carnivores but also with the
insect eaters, the marsupials, and the hoofed mammals as well.
From these primitive flesh eaters, with small and simple brains,
numerous small teeth, and plantigrade tread, the different
families of the carnivores of the present have slowly evolved.

DOGS AND BEARS. The dog family diverged from the creodonts late in
the Eocene, and divided into two branches, one of which evolved
the wolves and the other the foxes. An offshoot gave rise to the
family of the bears, and so closely do these two families, now
wide apart, approach as we trace them back in Tertiary times that
the Amphicyon, a genus doglike in its teeth and bearlike in other
structures, is referred by some to the dog and by others to the
bear family. The well-known plantigrade tread of bears is a
primitive characteristic which has survived from their creodont

CATS. The family of the cats, the most highly specialized of all
the carnivores, divided in the Tertiary into two main branches.
One, the saber-tooth tigers (Fig. 351), which takes its name from
their long, saberlike, sharp-edged upper canine teeth, evolved a
succession of genera and species, among them some of the most
destructive beasts of prey which ever scourged the earth. They
were masters of the entire northern hemisphere during the middle
Tertiary, but in Europe during the Pliocene they declined, from
unknown causes, and gave place to the other branch of cats,--which
includes the lions, tigers, and leopards. In the Americas the
saber-tooth tigers long survived the epoch.

MARINE MAMMALS. The carnivorous mammals of the sea--whales,
seals, walruses, etc.--seem to have been derived from some of the
creodonts of the early Tertiary by adaptation to aquatic life.
Whales evolved from some land ancestry at a very early date in the
Tertiary; in the marine deposits of the Eocene are found the bones
of the Zeuglodon, a whalelike creature seventy feet in length.

PRIMATES. This order, which includes lemurs, monkeys, apes, and
man, seems to have sprung from a creodont or insectivorous
ancestry in the lower Eocene. Lemur-like types, with small, smooth
brains, were abundant in the United States in the early Tertiary,
but no primates have been found here in the middle Tertiary and
later strata. In Europe true monkeys were introduced in the
Miocene, and were abundant until the close of the Tertiary, when
they were driven from the continent by the increasing cold.

millions of years comprised in Tertiary time the mammals evolved
from the lowly, simple types which tenanted the earth at the
beginning of the period, into the many kinds of highly specialized
mammals of the Pleistocene and the present, each with the various
structures of the body adapted to its own peculiar mode of life.
The swift feet of the horse, the horns of cattle and the antlers
of the deer, the lion's claws and teeth, the long incisors of the
beaver, the proboscis of the elephant, were all developed in
Tertiary times. In especial the brain of the Tertiary mammals
constantly grew larger relatively to the size of body, and the
higher portion of the brain--the cerebral lobes--increased in size
in comparison with the cerebellum. Some of the hoofed mammals now
have a brain eight or ten times the size of that of their early
Tertiary predecessors of equal bulk. Nor can we doubt that along
with the increasing size of brain went a corresponding increase in
the keenness of the senses, in activity and vigor, and in



The last period of geological history, the Quaternary, may be said
to have begun when all, or nearly all, living species of mollusks
and most of the existing mammals had appeared.

It is divided into two great epochs. The first, the Pleistocene or
Glacial epoch, is marked off from the Tertiary by the occupation
of the northern parts of North America and Europe by vast ice
sheets; the second, the Recent epoch, began with the disappearance
of the ice sheets from these continents, and merges into the
present time.


We now come to an episode of unusual interest, so different was it
from most of the preceding epochs and from the present, and so
largely has it influenced the conditions of man's life.

The records of the Glacial epoch are so plain and full that we are
compelled to believe what otherwise would seem almost incredible,
--that following the mild climate of the Tertiary came a succession
of ages when ice fields, like that of Greenland, shrouded the
northern parts of North America and Europe and extended far into
temperate latitudes.

THE DRIFT. Our studies of glaciers have prepared us to decipher
and interpret the history of the Glacial epoch, as it is recorded
in the surface deposits known as the drift. Over most of Canada
and the northern states this familiar formation is exposed to view
in nearly all cuttings which pass below the surface soil. The
drift includes two distinct classes of deposits,--the unstratified
drift laid down by glacier ice, and the stratified drift spread by
glacier waters.

The materials of the drift are in any given place in part unlike
the rock on which it rests. They cannot be derived from the
underlying rock by weathering, but have been brought from
elsewhere. Thus where a region is underlain by sedimentary rocks,
as is the drift-covered area from the Hudson River to the
Missouri, the drift contains not only fragments of limestone,
sandstone, and shale of local derivation, but also pebbles of many
igneous and metamorphic rocks, such as granites, gneisses,
schists, dike rocks, quartzites, and the quartz of mineral veins,
whose nearest source is the Archean area of Canada and the states
of our northern border. The drift received its name when it was
supposed that the formation had been drifted by floods and
icebergs from outside sources,--a theory long since abandoned.

The distribution also of the drift points clearly to its peculiar
origin. Within the limits of the glaciated area it covers the
country without regard to the relief, mantling with its debris not
only lowlands and valleys but also highlands and mountain slopes.

The boundary of the drift is equally independent of the relief of
the land, crossing hills and plains impartially, unlike water-laid
deposits, whose margins, unless subsequently deformed, are
horizontal. The boundary of the drift is strikingly lobate also,
bending outward in broad, convex curves, where there are no
natural barriers in the topography of the country to set it such a
limit. Under these conditions such a lobate margin cannot belong
to deposits of rivers, lakes, or ocean, but is precisely that
which would mark the edge of a continental glacier which deployed
in broad tongues of ice.

drift rests on firm, fresh rock, showing that both the preglacial
mantle of residual waste and the partially decomposed and broken
rock beneath it have been swept away. The underlying rock,
especially if massive, hard, and of a fine grain, has often been
ground down to a smooth surface and rubbed to a polish as perfect
as that seen on the rock beside an Alpine glacier where the ice
has recently melted back. Frequently it has been worn to the
smooth, rounded hummocks known as roches moutonnees, and even
rocky hills have been thus smoothed to flowing outlines like
roches moutonnees on a gigantic scale. The rock pavement beneath
the drift is also marked by long, straight, parallel scorings,
varying in size from deep grooves to fine striae as delicate as
the hair lines cut by an engraver's needle. Where the rock is soft
or closely jointed it is often shattered to a depth of several
feet beneath the drift, while stony clay has been thrust in among
the fragments into which the rock is broken.

In the presence of these glaciated surfaces we cannot doubt that
the area of the drift has been overridden by vast sheets of ice
which, in their steady flow, rasped and scored the rock bed
beneath by means of the stones with which their basal layers were
inset, and in places plucked and shattered it.

TILL. The unstratified portion of the drift consists chiefly of
sheets of dense, stony clay called till, which clearly are the
ground moraines of ancient continental glaciers. Till is an
unsorted mixture of materials of all sizes, from fine clay and
sand, gravel, pebbles, and cobblestones, to large bowlders. The
stones of the till are of many kinds, some having been plucked
from the bed rock of the locality where they are found, and others
having been brought from outside and often distant places. Land
ice is the only agent known which can spread unstratified material
in such extensive sheets.

The FINE MATERIAL of the till comes from two different sources. In
part it is derived from old residual clays, which in the making
had been leached of the lime and other soluble ingredients of the
rock from which they weathered. In part it consists of sound rock
ground fine; a drop of acid on fresh, clayey till often proves by
brisk effervescence that the till contains much undecayed
limestone flour. The ice sheet, therefore, both scraped up the
mantle of long-weathered waste which covered the coun try before
its coming, and also ground heavily upon the sound rock
underneath, and crushed and wore to rock flour the fragments which
it carried.

The color of unweathered till depends on that of the materials of
which it is composed. Where red sandstones have contributed
largely to its making, as over the Triassic sandstones of the
eastern states and the Algonkian sandstones about Lake Superior,
the drift is reddish. When derived in part from coaly shales, as
over many outcrops of the Pennsylvanian, it may when moist be
almost black. Fresh till is normally a dull gray or bluish, so
largely is it made up of the grindings of unoxidized rocks of
these common colors.

Except where composed chiefly of sand or coarser stuff,
unweathered till is often exceedingly dense. Can you suggest by
what means it has been thus compacted? Did the ice fields of the
Glacial epoch bear heavy surface moraines like the medial and
lateral moraines of valley glaciers? Where was the greater part of
the load of these ice fields carried, judging from what you know
of the glaciers of Greenland?

BOWLDERS OF THE DRIFT. The pebbles and bowlders of the drift are
in part stream gravels, bowlders of weathering, and other coarse
rock waste picked up from the surface of the country by the
advancing ice, and in part are fragments plucked from ledges of
sound rock after the mantle of waste had been removed. Many of the
stones of the till are dressed as only glacier ice can do; their
sharp edges have been blunted and their sides faceted and scored.

We may easily find all stages of this process represented among
the pebbles of the till. Some are little worn, even on their
edges; some are planed and scored on one side only; while some in
their long journey have been ground down to many facets and have
lost much of their original bulk. Evidently the ice played fast
and loose with a stone carried in its basal layers, now holding it
fast and rubbing it against the rock beneath, now loosening its
grasp and allowing the stone to turn.

Bowlders of the drift are sometimes found on higher ground than
their parent ledges. Thus bowlders have been left on the sides of
Mount Katahdin, Maine, which were plucked from limestone ledges
twelve miles distant and three thousand feet lower than their
resting place. In other cases stones have been carried over
mountain ranges, as in Vermont, where pebbles of Burlington red
sandstone were dragged over the Green Mountains, three thousand
feet in height, and left in the Connecticut valley sixty miles
away. No other geological agent than glacier ice could do this

The bowlders of the drift are often large. Bowlders ten and twenty
feet in diameter are not uncommon, and some are known whose
diameter exceeds fifty feet. As a rule the average size of
bowlders decreases with increasing distance from their sources.

TILL PLAINS. The surface of the drift, where left in its initial
state, also displays clear proof of its glacial origin. Over large
areas it is spread in level plains of till, perhaps bowlder-
dotted, similar to the plains of stony clay left in Spitzbergen by
the recent retreat of some of the glaciers of that island. In
places the unstratified drift is heaped in hills of various kinds,
which we will now describe.

DRUMLINS. Drumlins are smooth, rounded hills composed of till,
elliptical in base, and having their longer axes parallel to the
movement of the ice as shown by glacial scorings. They crowd
certain districts in central New York and in southern Wisconsin,
where they may be counted by the thousands. Among the numerous
drumlins about Boston is historic Bunker Hill.

Drumlins are made of ground moraine. They were accumulated and
given shape beneath the overriding ice, much as are sand bars in a
river, or in some instances were carved, like roches moutonnees,
by an ice sheet out of the till left by an earlier ice invasion.

TERMINAL MORAINES. The glaciated area is crossed by belts of
thickened drift, often a mile or two, and sometimes even ten miles
and more, in breadth, which lie transverse to the movement of the
ice and clearly are the terminal moraines of ancient ice sheets,
marking either the limit of their farthest advance or pauses in
their general retreat.

The surface of these moraines is a jumble of elevations and
depressions, which vary from low, gentle swells and shallow sags
to sharp hills, a hundred feet or so in height, and deep, steep-
sided hollows. Such tumultuous hills and hummocks, set with
depressions of all shapes, which usually are without outlet and
are often occupied by marshes, ponds, and lakes, surely cannot be
the work of running water. The hills are heaps of drift, lodged
beneath the ice edge or piled along its front. The basins were
left among the tangle of morainic knolls and ridges as the margin
of the ice moved back and forth. Some bowl-shaped basins were made
by the melting of a mass of ice left behind by the retreating
glacier and buried in its debris.

THE STRATIFIED DRIFT. Like modern glaciers the ice sheets of the
Pleistocene were ever being converted into water about their
margins. Their limits on the land were the lines where their
onward flow was just balanced by melting and evaporation. On the
surface of the ice along the marginal zone, rivulets no doubt
flowed in summer, and found their way through crevasses to the
interior of the glacier or to the ground. Subglacial streams, like
those of the Malaspina glacier, issued from tunnels in the ice,
and water ran along the melting ice front as it is seen to do
about the glacier tongues of Greenland. All these glacier waters
flowed away down the chief drainage channels in swollen rivers
loaded with glacial waste.

It is not unexpected therefore that there are found, over all the
country where the melting ice retreated, deposits made of the same
materials as the till, but sorted and stratified by running water.
Some of these were deposited behind the ice front in ice-walled
channels, some at the edge of the glaciers by issuing streams, and
others were spread to long distances in front of the ice edge by
glacial waters as they flowed away.

ESKERS are narrow, winding ridges of stratified sand and gravel
whose general course lies parallel with the movement of the
glacier. These ridges, though evidently laid by running water, do
not follow lines of continuous descent, but may be found to cross
river valleys and ascend their sides. Hence the streams by which
eskers were laid did not flow unconfined upon the surface of the
ground. We may infer that eskers were deposited in the tunnels and
ice-walled gorges of glacial streams before they issued from the
ice front.

KAMES are sand and gravel knolls, associated for the most part
with terminal moraines, and heaped by glacial waters along the
margin of the ice.

KAME TERRACES are hummocky embankments of stratified drift
sometimes found in rugged regions along the sides of valleys. In
these valleys long tongues of glacier ice lay slowly melting.
Glacial waters took their way between the edges of the glaciers
and the hillside, and here deposited sand and gravel in rude

Outwash plains are plains of sand and gravel which frequently
border terminal moraines on their outward face, and were spread
evidently by outwash from the melting ice. Outwash plains are
sometimes pitted by bowl-shaped basins where ice blocks were left
buried in the sand by the retreating glacier.

Valley trains are deposits of stratified drift with which river
valleys have been aggraded. Valleys leading outward from the ice
front were flooded by glacial waters and were filled often to
great depths with trains of stream-swept drift. Since the
disappearance of the ice these glacial flood plains have been
dissected by the shrunken rivers of recent times and left on
either side the valley in high terraces. Valley trains head in
morainic plains, and their material grows finer down valley and
coarser toward their sources. Their gradient is commonly greater
than that of the present rivers.

THE EXTENT OF THE DRIFT. The extent of the drift of North America
and its southern limits are best seen in Figure 359. Its area is
reckoned at about four million square miles. The ice fields which
once covered so much of our continent were all together ten times
as large as the inland ice of Greenland, and about equal to the
enormous ice cap which now covers the antartic regions.

The ice field of Europe was much smaller, measuring about seven
hundred and seventy thousand square miles.

CENTERS OF DISPERSION. The direction of the movement of the ice is
recorded plainly in the scorings of the rock surface, in the
shapes of glaciated hills, in the axes of drumlins and eskers, and
in trains of bowlders, when the ledges from which they were
plucked can be discovered. In these ways it has been proved that
in North America there were three centers where ice gathered to
the greatest depth, and from which it flowed in all directions
outward. There were thus three vast ice fields,--one the
Cordilleran, which lay upon the Cordilleras of British America;
one the Keewatin, which flowed out from the province of Keewatin,
west of Hudson Bay; and one the LABRADOR ice field, whose center
of dispersion was on the highlands of the peninsula of Labrador.
As shown in Figure 359, the western ice field extended but a short
way beyond the eastern foothills of the Rocky Mountains, where
perhaps it met the far-traveled ice from the great central field.
The Keewatin and the Labrador ice fields flowed farthest toward
the south, and in the Mississippi valley the one reached the mouth
of the Missouri and the other nearly to the mouth of the Ohio. In
Minnesota and Wisconsin and northward they merged in one vast

The thickness of the ice was so great that it buried the highest
mountains of eastern North America, as is proved by the
transported bowlders which have been found upon their summits. If
the land then stood at its present height above sea level, and if
the average slope of the ice were no more than ten feet to the
mile,--a slope so gentle that the eye could not detect it and less
than half the slope of the interior of the inland ice of
Greenland,--the ice plateaus about Hudson Bay must have reached a
thickness of at least ten thousand feet.

In Europe the Scandinavian plateau was the chief center of
dispersion. At the time of greatest glaciation a continuous field
of ice extended from the Ural Mountains to the Atlantic, where,
off the coasts of Norway and the British Isles, it met the sea in
an unbroken ice wall. On the south it reached to southern England,
Belgium, and central Germany, and deployed on the eastern plains
in wide lobes over Poland and central Russia (Fig. 360).

At the same time the Alps supported giant glaciers many times the
size of the surviving glaciers of to-day, and a piedmont glacier
covered the plains of northern Switzerland.

THE THICKNESS OF THE DRIFT. The drift is far from uniform in
thickness. It is comparatively thin and scanty over the Laurentian
highlands and the rugged regions of New England, while from
southern New York and Ontario westward over the Mississippi
valley, and on the great western plains of Canada, it exceeds an
average of one hundred feet over wide areas, and in places has
five and six times that thickness. It was to this marginal belt
that the ice sheets brought their loads, while northwards, nearer
the centers of dispersion, erosion was excessive and deposition

the drift prove that it does not consist of one indivisible
formation, but includes a number of distinct drift sheets, each
with its own peculiar features. The Pleistocene epoch consisted,
therefore, of several glacial stages,--during each of which the
ice advanced far southward,--together with the intervening
interglacial stages when, under a milder climate, the ice melted
back toward its sources or wholly disappeared.

The evidences of such interglacial stages, and the means by which
the different drift sheets are told apart, are illustrated in
Figure 361. Here the country from N to S is wholly covered by
drift, but the drift from N to m is so unlike that from m to S
that we may believe it the product of a distinct ice invasion and
deposited during another and far later glacial stage. The former
drift is very young, for its drainage is as yet immature, and
there are many lakes and marshes upon its surface; the latter is
far older, for its surface has been thoroughly dissected by its
streams. The former is but slightly weathered, while the latter is
so old that it is deeply reddened by oxidation and is leached of
its soluble ingredients such as lime. The younger drift is
bordered by a distinct terminal moraine, while the margin of the
older drift is not thus marked. Moreover, the two drift sheets are
somewhat unlike in composition, and the different proportion of
pebbles of the various kinds of rocks which they contain shows
that their respective glaciers followed different tracks and
gathered their loads from different regions. Again, in places
beneath the younger drift there is found the buried land surface
of an older drift with old soils, forest grounds, and vegetable
deposits, containing the remains of animals and plants, which tell
of the climate of the interglacial stage in which they lived.

By such differences as these the following drift sheets have been
made out in America, and similar subdivisions have been recognized
in Europe.

5 The Wisconsin formation
4 The Iowan formation
3 The Illinoian formation
2 The Kansan formation
1 The pre-Kansan or Jerseyan formation

In New Jersey and Pennsylvania the edge of a deeply weathered and
eroded drift sheet, the Jerseyan, extends beyond the limits of a
much younger overlying drift. It may be the equivalent of a deep-
buried basal drift sheet found in the Mississippi valley beneath
the Kansan and parted from it by peat, old soil, and gravel beds.

The two succeeding stages mark the greatest snowfall of the
Glacial epoch. In Kansan times the Keewatin ice field slowly grew
southward until it reached fifteen hundred miles from its center
of dispersion and extended from the Arctic Ocean to northeastern
Kansas. In the Illinoian stage the Labrador ice field stretched
from Hudson Straits nearly to the Ohio River in Illinois. In the
Iowan and the Wisconsin, the closing stages of the Glacial epoch,
the readvancing ice fields fell far short of their former limits
in the Mississippi valley, but in the eastern states the Labrador
ice field during Wisconsin times overrode for the most part all
earlier deposits, and, covering New England, probably met the
ocean in a continuous wall of ice which set its bergs afloat from
Massachusetts to northern Labrador.

We select for detailed description the Kansan and the Wisconsin
formations as representatives, the one of the older and the other
of the younger drift sheets.

THE KANSAN FORMATION. The Kansan drift consists for the most part
of a sheet of clayey till carrying smaller bowlders than the later
drift. Few traces of drumlins, kames, or terminal moraines are
found upon the Kansan drift, and where thick enough to mask the
preexisting surface, it seems to have been spread originally in
level plains of till.

The initial Kansan plain has been worn by running water until
there are now left only isolated patches and the narrow strips and
crests of the divides, which still rise to the ancient level. The
valleys of the larger streams have been opened wide. Their well-
developed tributaries have carved nearly the entire plain to
valley slopes (Figs. 50 B, and 59). The lakes and marshes which
once marked the infancy of the region have long since been
effaced. The drift is also deeply weathered. The till, originally
blue in color, has been yellowed by oxidation to a depth of ten
and twenty feet and even more, and its surface is sometimes rusted
to terra-cotta red. To a somewhat less depth it has been leached
of its lime and other soluble ingredients. In the weathered zone
its pebbles, especially where the till is loose in texture, are
sometimes so rotted that granites may be crumbled with the
fingers. The Kansan drift is therefore old.

THE WISCONSIN FORMATION. The Wisconsin drift sheet is but little
weathered and eroded, and therefore is extremely young. Oxidation
has effected it but slightly, and lime and other soluble plant
foods remain undissolved even at the grass roots. Its river
systems are still in their infancy (Fig. 50, A). Swamps and peat
bogs are abundant on its undrained surface, and to this drift
sheet belong the lake lands of our northern states and of the
Laurentian peneplain of Canada.

The lake basins of the Wisconsin drift are of several different
classes. Many are shallow sags in the ground moraine. Still more
numerous are the lakes set in hollows among the hills of the
terminal moraines; such as the thousands of lakelets of eastern
Massachusetts. Indeed, the terminal moraines of the Wisconsin
drift may often be roughly traced on maps by means of belts of
lakes and ponds. Some lakes are due to the blockade of ancient
valleys by morainic delms, and this class includes many of the
lakes of the Adirondacks, the mountain regions of New England,
and the Laurentian area. Still other lakes rest in rock basins
scooped out by glaciers. In many cases lakes are due to more than
one cause, as where preglacial valleys have both been basined by
the ice and blockaded by its moraines. The Finger lakes of New
York, for example, occupy such glacial troughs.

Massive TERMINAL MORAINES, which mark the farthest limits to which
the Wisconsin ice advanced, have been traced from Cape Cod and the
islands south of New England, across the Appalachians and the
Mississippi valley, through the Dakotas, and far to the north over
the plains of British America. Where the ice halted for a time in
its general retreat, it left RECESSIONAL MORAINES, as this variety
of the terminal moraine is called. The moraines of the Wisconsin
drift lie upon the country like great festoons, each series of
concentric loops marking the utmost advance of broad lobes of the
ice margin and the various pauses in their recession.

Behind the terminal moraines lie wide till plains, in places
studded thickly with drumlins, or ridged with an occasional esker.
Great outwash plains of sand and gravel lie in front of the
moraine belts, and long valley trains of coarse gravels tell of
the swift and powerful rivers of the time.

THE LOESS OF THE MISSISSIPPI VALLEY. A yellow earth, quite like
the loess of China, is laid broadly as a surface deposit over the
Mississippi valley from eastern Nebraska to Ohio outside the
boundaries of the Iowan and the Wisconsin drift. Much of the loess
was deposited in Iowan times. It is younger than the earlier drift
sheets, for it overlies their weathered and eroded surfaces. It
thickens to the Iowan drift border, but is not found upon that
drift. It is older than the Wisconsin, for in many places it
passes underneath the Wisconsin terminal moraines. In part the
loess seems to have been washed from glacial waste and spread in
sluggish glacial waters, and in part to have been distributed by
the wind from plains of aggrading glacial streams.

invasions of the Pleistocene profoundly disarranged the drainage
systems of our northern states. In some regions the ancient
valleys were completely filled with drift. On the withdrawal of
the ice the streams were compelled to find their way, as best they
could, over a fresh land surface, where we now find them flowing
on the drift in young, narrow channels. But hundreds of feet below
the ground the well driller and the prospector for coal and oil
discover deep, wide, buried valleys cut in rock,--the channels of
preglacial and interglacial streams. In places the ancient valleys
were filled with drift to a depth of a hundred feet, and sometimes
even to a depth of four hundred and five hundred feet. In such
valleys, rivers now flow high above their ancient beds of rock on
floors of valley drift. Many of the valleys of our present rivers
are but patchworks of preglacial, interglacial, and postglacial
courses (Fig. 366). Here the river winds along an ancient valley
with gently sloping sides and a wide alluvial floor perhaps a mile
or so in width, and there it enters a young, rock-walled gorge,
whose rocky bed may be crossed by ledges over which the river
plunges in waterfalls and rapids.

In such cases it is possible that the river was pushed to one side
of its former valley by a lobe of ice, and compelled to cut a new
channel in the adjacent uplands. A section of the valley may have
been blockaded with morainic waste, and the lake formed behind the
barrier may have found outlet over the country to one side of the
ancient drift-filled valley. In some instances it would seem that
during the waning of the ice sheets, glacial streams, while
confined within walls of stagnant ice, cut down through the ice
and incised their channels on the underlying country, in some
cases being let down on old river courses, and in other cases
excavating gorges in adjacent uplands.

PLEISTOCENE LAKES. Temporary lakes were formed wherever the ice
front dammed the natural drainage of the region. Some, held in the
minor valleys crossed by ice lobes, were small, and no doubt many
were too short-lived to leave lasting records. Others, long held
against the northward sloping country by the retreating ice edge,
left in their beaches their clayey beds, and their outlet channels
permanent evidences of their area and depth. Some of these glacial
lakes are thus known to have been larger than any present lake.

Lake Agassiz, named in honor of the author of the theory of
continental glaciation, is supposed to have been held by the
united front of the Keewatin and the Labrador ice fields as they
finally retreated down the valley of the Red River of the North
and the drainage basin of Lake Winnipeg. From first to last Lake
Agassiz covered a hundred and ten thousand square miles in
Manitoba and the adjacent parts of Minnesota and North Dakota,--an
area larger than all the Great Lakes combined. It discharged its
waters across the divide which held it on the south, and thus
excavated the valley of the Minnesota River. The lake bed--a plain
of till--was spread smooth and level as a floor with lacustrine
silts. Since Lake Agassiz vanished with the melting back of the
ice beyond the outlet by the Nelson River into Hudson Bay, there
has gathered on its floor a deep humus, rich in the nitrogenous
elements so needful for the growth of plants, and it is to this
soil that the region owes its well-known fertility.

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