MEDIEVAL EARTH HISTORY
(_Mesozoic Era_)
What was the condition of North America during the first or Triassic
period of the Mesozoic era, approximately 8 or 10 million years ago?
As a result of the Appalachian Revolution the sea was excluded from
all the land except along much of the western side from southern
California to parts of Alaska. On this western side of the continent
the Appalachian Revolution had little or no effect and the Permian
conditions continued, essentially without change through the Triassic.
The Triassic strata up to 4,000 feet thick are there of typical marine
origin. In British Columbia and Alaska there was much igneous activity.
Throughout much of the Rocky Mountains and Great Plains region of the
western United States there are extensive deposits of red sediments
(so-called "Red Beds"), containing layers of salt and gypsum, from 200
to 1,000 or more feet thick. These strata commonly rest in regular
order on Permian Red Beds, so that conditions of deposition of Permian
time continued through Triassic time, that is continental deposits
formed mostly in salt lakes, fresh lakes, along stream courses, and on
land in part by the action of wind.
[Illustration: Fig. 40.--Map showing the general relations of land
and water in North America during the Triassic period. Lined areas
represent land; vertical-lined areas, basins in which continental
deposits formed. (Based upon map by Willis; courtesy of the Journal of
Geology.)]
In the eastern half of North America there is no record of accumulation
of any marine strata whatever, because, as a result of the Appalachian
Revolution, the land was brought well above sea level. There was,
however, deposition of a remarkable series of nonmarine strata in
several long, narrow, troughlike depressions whose trend was parallel
to, and just east of, the main axis of the newly formed Appalachian
Range. These troughs lay between the Appalachians and the very
persistent old land mass called Appalachia which we have already
described. The facts that these troughs are truly down-warps; that
they so perfectly follow the trend of the Appalachian Mountain folds;
and that the strata in them are of late Triassic Age, make it certain
that they were formed by a great lateral pressure which must have been
a continuation of the Appalachian Revolution. Thus the Appalachian
Mountains continued to grow well into the Triassic period, and, while
the Paleozoic strata were being folded, the surface of old Appalachia
(including part of the Taconic Mountain region) was down-warped to
form the troughs in which the late Triassic strata accumulated. One
trough extended through the Connecticut Valley; another (the largest)
from southeastern New York through northern New Jersey, southeastern
Pennsylvania, Maryland, and into Virginia; while several smaller ones
occurred in Virginia and North Carolina.
The down-warps or troughlike basins were very favorably situated
for rapid accumulation of thick sedimentary deposits because of
their position just between large, high land masses which were being
vigorously eroded. The sediments derived from the erosion of the young
Appalachians were especially abundant because of the vigorous wearing
down of the newly formed high mountains. A thickness of from 5,000
to fully 15,000 feet of mostly red sandstones and shales accumulated
in these down-warps, the character and great thickness of the strata
strongly pointing to gradual down-warping as the deposition of the
sediments went on. It is often stated that these strata were formed in
estuaries, but, in the northern areas, at least from Massachusetts to
Maryland, many of the layers show ripple marks, sun cracks, rain-drop
pits, fossil plants, and fossil bones and tracks of land reptiles. Such
strata may well have formed in very shallow water, such as river-flood
plains or temporary lakes, where changing conditions frequently allowed
the surface layers to lie exposed to the sun.
[Illustration: Fig. 41.--Block diagram of the region westward from
New York City and vicinity, showing the main relief features, the
underground relations of rocks of widely different ages, and the
relation of the relief to the rock formations. (Part of larger drawing
by A. K. Lobeck.)]
During the time of the accumulation of the late Triassic strata in the
down-warp basins there was considerable igneous activity, as proved by
the occurrence of sheets of igneous rock within the body of strata. In
some cases true lava flows with cindery tops were forced out on the
surface and then buried under later sediments, while in other cases
the sheets of molten rock were forced up either between the strata or
obliquely through them, thus proving their intrusive character. As a
result of subsequent erosion, these very resistant lava masses often
stand out conspicuously as relief features. Perhaps the most noteworthy
example is the great layer of such intrusive igneous rock, part of
which outcrops for seventy miles mostly as a bold cliff forming the
famous Palisades of the Hudson, near New York City. During the process
of cooling and solidification of the molten mass there was contraction
which expressed itself by breaking the rock mass into great, crude,
nearly vertical columns, and hence the origin of the name "Palisades."
The cliff character of the outcrop is due to the fact that the lava
is much more resistant to erosion than the sandstone above and below
it. In the Connecticut Valley of Massachusetts a layer of lava several
hundred feet thick boldly outcrops, forming the crest of the well-known
Mount Tom-Mount Holyoke Range.
The close of the Triassic period was marked by enough uplift to leave
the whole eastern two-thirds of the continent dry land undergoing
erosion. The Triassic deposits of the Atlantic Coast are much broken up
into large fault blocks, and this faulting probably took place as a
result of the crustal disturbances toward the end of the period. In the
west the Triassic conditions seem to have continued without much change
into the next (or Jurassic) period.
During the Jurassic period the relations of land and water in North
America were very simple. In the earlier Jurassic all was dry land
except portions of the western fringe of the continent from southern
California to Alaska, where marine strata 2,000 to 10,000 feet thick
accumulated. Late in the period the conditions were the same, except
for a long, narrow arm of the sea or mediterranean which extended from
the Arctic Ocean southward across the site of the Rocky Mountains to
Arizona. There is no evidence for the existence of anything like real
mountains anywhere on the continent during the period.
[Illustration: Fig. 42.--Structure section showing profile and
underground relations of rocks across the Connecticut Valley (through
Mount Tom) of Massachusetts. Js and Jl are sandstone strata, with
included lava sheets (in black) resting upon Paleozoic rocks on either
side. The rocks have been notably tilted and faulted. (After Emerson,
U. S. Geological Survey.)] Profound crustal disturbances marked the
close of the Jurassic period in the western part of the continent.
Strata which had accumulated to great thickness during millions
of years of time, mainly over the sites of the Sierra Nevada and
Cascade Mountains, finally yielded to a tremendous force of lateral
compression, especially in the Sierra region, and were folded,
crumpled, and upraised. Thus the Sierra-Cascade district was originally
built up into a high mountain range. Since that time the Sierras
have been much cut down by erosion and they have been rejuvenated
by faulting and tilting of the great earth block. The Cascade Range
from northern California into British Columbia was apparently not so
profoundly raised, and its present height is mainly due to subsequent
volcanic activity. The rocks of the Klamath Mountains of northwestern
California, and of the Humboldt Range of Nevada, were also folded at
that time.
During the mountain-making disturbances on the western side of the
continent great quantities of molten granite were forced up into the
lower portions of the folding strata. Because of profound subsequent
erosion this granite is now widely exposed as, for example, in the
great walls of the Yosemite Valley.
During the earlier half of the last period (the Cretaceous) of the
Mesozoic era, sea water spread from Mississippi northwestward to the
site of Denver and southward over Texas and much of Mexico. At the
same time much of the western margin of the continent from Alaska to
California was submerged. All the rest of the continent was land.
During this time sediments accumulated on low lands just east of the
site of the present Rocky Mountains, and also east of the Appalachians,
as proved by the numerous fossils of land plants found in these
deposits.
[Illustration: Fig. 43.--Map Showing the general rotations of land
and water in North America during later Cretaceous time, several
million years ago. Lined areas represent land; vertical lines, mainly
continental deposits. (Principal data from a map by Willis, published
in the Journal of Geology.)]
As Cretaceous time went on the marine waters gradually spread until
the whole Atlantic and Gulf coastal plain regions from Long Island,
New York, to Mexico became submerged under marine water, and a wide
arm of the sea, or great mediterranean, spread from Texas north to the
mouth of the Mackenzie River. The Gulf of Mexico was thus directly
connected with the Arctic Ocean. This great interior sea was nowhere
connected with the Pacific Ocean, though portions of the Pacific
border of the continent were submerged. This vast interior sea was not
only the largest of any which reached well into the continent since
the Mississippian period of the Paleozoic era, but it was the last
body of marine water which ever extended well into the continent. It
should be stated that the later Cretaceous was also a time of unusually
widespread submergence of the continents, when most of southern Europe
and southeastern Asia, as well as about one-half of both Africa and
South America were submerged. Over much of the site of the Rocky
Mountains during the late Cretaceous there were low lands receiving
continental deposits, and extensive marshes supporting prolific
vegetation were common. Much of this vegetable matter became buried,
and has since been converted into workable coal.
The maximum thickness of strata accumulated during all of Cretaceous
time over the Atlantic coastal plain area was about 1,700 feet; over
the Gulf coastal plain region fully 7,500 feet; over the western
interior 10,000 to 15,000; and over parts of the Pacific border 25,000
to 30,000 feet, as in California. The last-named figures are truly
phenomenal, representing a thickness about equal to the total thickness
of all the strata accumulated during the whole Paleozoic era (seven
periods) and piled up in the Appalachian Mountain region. This great
deposit of strata of mostly early Cretaceous Age is readily accounted
for when we realize that these sediments, which accumulated in the
marginal sea bottom, were derived from the very rapidly eroding, newly
formed lofty Sierra Nevada Range.
[Illustration: Fig. 44.--Sketch of a mountain range along Skolat Creek,
Alaska, showing Tertiary lava beds resting upon deeply eroded tilted
limestones and lavas of late Paleozoic (Carboniferous) Age. The present
topography has been produced by erosion since the Tertiary lavas flowed
out. (After U. S. Geological Survey.)]
Especially in Alabama and Texas the Cretaceous system is remarkable for
its richness in chalk deposits. In Alabama a widespread formation of
late Cretaceous Age, about 1,000 feet thick, contains much nearly pure
white chalk, and in Texas a similarly constituted formation of early
middle Cretaceous Age is from 1,000 to 5,000 feet thick. These chalk
deposits consist almost wholly of carbonate of lime shells or very
tiny single-celled animals which accumulated under exceptionally clear
sea water which spread over those parts of Alabama and Texas where the
chalk now occurs. Here again we have a bit of evidence supporting the
fact of very long geologic time. Think of how long it must have taken
for the tiny (even microscopic) shells to form a widespread layer of
chalk nearly a mile thick!
The close of the Cretaceous period, or what is the same, the close
of the Mesozoic era, was marked by some of the grandest crustal
disturbances in the known history of the earth. In fact, it is not
known that the western hemisphere was ever affected by more profound
and widespread mountain-making disturbances than those which took
place toward the close of the Mesozoic era, and continued into the
succeeding Tertiary period. These disturbances were of three kinds:
folding of strata, volcanic activity, and renewed uplift of old
mountains without folding of the rocks. Greatest of all was the "Rocky
Mountain Revolution," during which the thick strata, which accumulated
during the Paleozoic and Mesozoic eras over the site of the Rockies,
yielded to vigorous deformation when they were more or less folded
and dislocated from Alaska to Central America. This was in truth the
birth of the Rocky Mountains, although their existing altitude and
configuration have, to a very considerable degree, resulted from later
uplift and erosion. In the northern United States and southern Canada
the Rocky Mountain strata, up to over 40,000 feet thick, were most
severely folded and fractured, forming a range which quite certainly
was fully 20,000 feet high. In this district a great thrust fault,
hundreds of miles long, developed, and rocks as old as the Proterozoic
were shoved at least seven miles, and probably as much as twenty
miles, westward, over Cretaceous and other rocks much later than the
Proterozoic. At the same time the Andes Mountains throughout South
America were notably upraised and the rocks folded.
The second type of physical disturbance was volcanic activity which
took place on a tremendous scale, and which appears to have started as
a direct accompaniment of the Rocky Mountain Revolution. This igneous
activity took place not only in the Rocky Mountains but also westward
to and in the Sierra-Cascade Range, as well as in the mountains of
western British Columbia and Alaska. This activity continued well into
the succeeding Cenozoic era, and it is more fully considered in the
next chapter.
The third type of crustal disturbance took place on a large scale when
the Appalachian Mountains, which had been almost wholly planed away by
erosion during Mesozoic time, were reelevated from 1,000 to 3,000 feet
by an uplifting force not accompanied by folding. All or nearly all
of New York and New England, as well as much of southeastern Canada,
were similarly upraised at the same time. This notable uplift of so
much of eastern North America is a matter of great importance because
the major relief features of that area have been produced by erosion
or dissection of the upraised surface since late Mesozoic or early
Cenozoic time. In view of the fact that this work of erosion took
place almost wholly during the Cenozoic era, it will be discussed in
the next chapter.
In conclusion, brief mention may be made of the kind of climate of the
Mesozoic era. As shown by the character and distribution of fossil
plants and animals, the Mesozoic climate was in general mild and rather
uniform over the earth, but with some distinction of climatic zones.
Such distinction of climatic zones is unknown for the Paleozoic era,
while it was notably less than at present.