Elements of Geology — Ch. XX: The Mesozoic, pt 1

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.

THE TRIASSIC AND JURASSIC

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 CRETACEOUS

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 Columbia.

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 deposit.

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.