Chapter 8

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 AppalachianMountain 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.

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 faultingprobably 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.

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.

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 ofthe 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.

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 erosiontook 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.

CHAPTER XVI

MODERN EARTH HISTORY

(Cenozoic Era)

SINCE the Cenozoic era is the last one of geologic time, it will be of particular interest to trace out the main events which have led up to the present day conditions, especially in North America. Both because of the recency of the time and the unusual accessibility of the rocks, which are mostly at or near the surface, our knowledge of the Cenozoic era is exceptionally detailed and accurate. It will, therefore, be more necessary than ever to select only the very significant features of this history for our brief discussion.

During the first half of the Tertiary period portions only of the Atlantic coastal plain were submerged under shallow water, but soon after the middle of the period (Miocene epoch) the sea spread over practically the whole Atlantic coastal plain area from Martha’s Vineyard south to and including Florida. During the late Tertiary the marine waters had become greatly restricted, and by the close of the period the sea was entirely excluded from the Atlantic seaboard. The total thickness of these Tertiary strata is less than 1,000 feet, and they all tilt downward gently toward the sea. The strata consist mostly of unconsolidated sands, gravels, clays, marls, etc.

The Gulf coastal plain area from Florida through Texas and south through eastern Mexico was largely overspread by the sea during most of Tertiary time, except the latest. During early Tertiary time an arm of the Gulf reached north to the mouth of the Ohio River. Late in the period but little of the Gulf Plain was submerged, and at its close sea water was wholly excluded. On the Gulf Coast the Tertiary strata from 2,000 to 4,000 feet thick are also mainly sands, gravels, clays, and marls. They are commonly rich in fossils, and they show a gentle tilt downward toward the Gulf.

Throughout Tertiary time local portions of the Pacific border of the continent were submerged, this having been especially true of portions of California, Oregon, and Washington. In spite of the very restricted marine waters, the Tertiary strata of the Pacific Coast, especially in California, are remarkably thick, 10,000 to 20,000 feet being common, while the maximum thickness is fully 30,000 feet. Such great thicknesses are readily explained when we realize that erosion was notably speeded up by pronounced uplifts resulting from crustal disturbances toward the close of the preceding period, and again in the midst of the Tertiary period itself.

To summarize the Tertiary relations of sea and land for North America we may say that only local portions of the continental border ever became submerged, and that, by late Tertiary time, practically the whole continent was a land area. At the close of the period the continent was, as we shall see, even larger than now because the continental shelves of the ocean were then also largely above water.

Fig. 45.—Map showing the general relations of land and water in North America during part of the middle Tertiary period. (After Willis, courtesy of the Journal of Geology.)

The whole of the Cenozoic era, including both the Tertiary and Quaternary periods, has been a time of profound crustal disturbances throughout much of the continent, certain of these movements having continued right up to the present time, with positive evidence that some of them are still continuing. These great movements have included notable foldings of strata, uplifts without folding, faulting, and igneous activity, the whole effect having been to greatly increase the general altitude and ruggedness of the continent. In fact, North America is notknown ever to have been at once higher, broader, and more rugged than it was very late in the Tertiary, or early in the Quaternary, period. Since that time the only notable change (barring the great Ice Age and its effects) has been a restriction of the area of the continent to its present size by spreading of sea waters over the borders of the continent, that is over the continental shelves.

We shall now rather systematically consider the more profound earth changes which have affected the continent, producing the existing major relief features, from west to east.

The “Coast Range Revolution” took place in the midst of the Tertiary period. Over the site of the Coast Ranges, strata had accumulated, especially during Cretaceous and earlier Tertiary times, to a thickness of thousands of feet. In middle Tertiary time these strata were subjected to a mountain-making force of compression and more or less folded, faulted (fractured), and uplifted into the Coast Range Mountains. Some portions of the range were intensely folded and faulted and upraised many thousands of feet, while other portions were only moderately folded and uplifted. It is an interesting fact that the great San Francisco earthquake rift or fault originated at this time. It was a renewed, sudden movement of a few feet along this fault which caused the disastrous earthquake of 1906. Still other considerable earth movements took place in the Coast Range region during late Tertiary and Quaternary times, as, for example, uplift without folding, as proved by distinct sea-cut terraces at altitudes of more than a thousand feet, like those north of San Francisco and south of Los Angeles. A moderate amount of still later subsidence has caused the development of San Francisco Bay. The large islands off the coast of southern California have in very recent geologic time (probably Quaternary) been cut off from the mainland by sinking of the land.

Plate11.—(a)Part of the Mammoth Hot Springs Terrace in the Yellowstone Park.The view shows the deposit with boiling water flowing over it. The water enters the earth back on the mountain, travels underground in contact with hot lava, rises through limestone, from which the boiling water takes into solution much carbonate of lime which is deposited when the water reaches the surface. (Photo by Jackson, U. S. Geological Survey.)

Plate11.—(b)View Across Part of Crater Lake, Oregon.This great hole, 3,000 to 4,000 feet deep and 6 miles in diameter and now partly filled with a lake 2,000 feet deep, was formed by a subsidence of the top of a once great cone-shaped volcano fully 14,000 feet high above the sea. The high rock in the distance rises 2,000 feet above the lake which is over 6,000 feet above sea level. The island is a small volcano of recent origin. (Photo by Russell, U. S. Geological Survey.)

Plate12.—(a)Detailed View of Part of the Very Oldest Known (Archeozoic) Rock Formation of the Earth.The rock is distinctly stratified and represents sands and muds deposited layer upon layer upon a sea floor at least 50,000,000 years ago. The sands and muds first consolidated into sandstone and shale below the earth’s surface. Then, under conditions of heat, moisture, and pressure, they were notably altered, mainly by crystallization of minerals, and raised high above sea level. Finally the strata were laid bare by erosion. (Photo by the author.)

Plate12.—(b)A Twisted Mass of Stratified Archeozoic Limestone Surrounded by Granite in Northern New York.The limestone was enveloped in the granite while it was being forced in molten condition into the earth’s crust. (Photo by the author.)

The Sierra Nevada Range, which originated by intense folding of rocks late in the Jurassic period, underwent profound erosion until about the middle of the Tertiary period, by which time it had been cut down to a range of hills or low mountains. Then the great fault (fracture) previously described began to develop along the eastern side. As a result of many sudden movements along this fault, which is hundreds of miles long, the vast earth block has been tilted westward with a very steep eastern face and a long, more gradual western slope, the crest of the fault block forming the summit of the range. The amount of nearly vertical displacement along this fault has been commonly from 10,000 to 20,000 feet, and, in spite of considerable erosion of the top of the fault block and accumulation of sediment at its eastern base, the modified fault face now usually stands out boldly from 2,000 to 10,000 feet high. As an evidence that this movement of faulting has not yet ceased we may cite the Inyo earthquake of 1872, when there was a sudden renewal of movement of ten to twenty-five feet along this fault for many miles. Since the great Sierra block began to tilt, the many mighty canyons, like Yosemite, Hetch-Hetchy, King’s River, and Feather River, have been carved out by the action of streams, in some cases aided by former glaciers. King’s River canyon has been sunk to a maximum depth of 6,900 feet in solid granite solely by the erosive action of the river!

The Cascade Mountains, too, were reduced to nearly a peneplain condition by late Tertiary time when they began to be rejuvenated by arching or bowing of the surface unaccompanied by great faulting or fracturing, and many canyons, like that of the Columbia River, have since been carved out.

Mention should now be made of the vigorous volcanic activity which took place in the Cascade and Sierra Nevada Ranges. Most of this activity occurred during Tertiary time (particularly in the latter part) and it has continued with diminishing force practically to the present time. In California streams of lava buried many gold-bearing river gravels which have yielded rich mines. Many well-known mountain peaks, such as Shasta, Lassen, Pitt, Hood, and Rainier, from northern California to Washington, are great volcanic cones which date from Tertiary time, and which are now mostly inactive. That this volcanic activity has not yet altogether ceased is shown by renewed eruptions of Mount Lassen (or Lassen Peak, altitude 10,437 feet) in northern California. Since the beginning of this renewed activity in 1914, several hundred outbursts have occurred. No molten rock has flowed out, but large quantities of rock fragments, dust and steam have been erupted, in many cases forming great clouds two or three miles high over the top of the mountain (Plate 10). At this writing (October, 1920), Mount Lassen is still showing vigorous activity. At Cinder Cone, only ten miles from Mount Lassen, there were two eruptions of cinders and a considerable outpouring of lava within the last 200 years. Still other very recent cinder cones occur in southeastern California and Arizona.

Fig. 46.—Sketch map showing the distribution of volcanic rocks of Cenozoic (mostly Tertiary) Age in western North America. Only one volcano (Mount Lassen, California) is now active in the United States proper, but a number are more or less active in Mexico and Central America. (Data from Willis, U. S. Geological Survey.)

One of the greatest lava fields in the world forms the Columbian Plateau between western Wyoming (including the Yellowstone National Park) and the Cascade Mountains from northeastern California to northern Washington. It covers fully 200,000 square miles and is really considerably larger than shown on the map because the lava in parts of theplateau region are covered by very recent sedimentary materials.

The great lava fields of the Deccan, India, and of the plateau region of western Mexico are comparable in size to the Columbian field and these lava fields are all of the same age. In the Columbian Plateau most of the lava was poured out during later Tertiary time. Sheets of molten rock, averaging fifty to one hundred feet in thickness, spread out over various parts of the region and piled up by overlapping layers one over another until the lava plateau more than a mile high was built up. Many hills and low mountains were completely buried under the molten floods, and in other places the liquid rock masses flowed against the higher mountains. “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 plateau was long in building. Between the layers are found in places old soil beds and forest grounds and the sediments of lakes.... So ancient are the latest floods in the Columbia River 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 (Quaternary). Their surfaces are so fresh and undecayed that here the effusive eruptions may have continued to within the period of human history.” (W. H. Norton.) Many of the lava layers are plainly visible where the Columbia River has cut its great gorge or canyon. The Snake River in places has sunk its channel several thousand feet into the lava plateau without reaching underlying rock.

Both north and south of the Columbian Plateau there was also much volcanic activity in the Rocky Mountain region during Tertiary time. A single formation in Colorado consists mostly of volcanic “ash” or dust over 2,000 feet thick. There was also much volcanic activity over the Colorado Plateau area of southern Utah, New Mexico, and Arizona. The volcanoes there exhibit all stages from those which are very recent and practically unaffected by erosion to others which have been completely cut away with the exception of the cores or “volcanic necks.”

During the second half of the Tertiary period the whole region known as the Great Basin, between the Sierra Nevada Mountains of California and the Wasatch Mountains of Utah, began to be affected by profound faulting or fracturing and tilting of portions of the earth’s crust. The two largest faults, one on the western side of the Wasatch Range and the other on the eastern side of the Sierra Range, are each hundreds of miles long. Each of these ranges owes most of its present altitude to the uptilting of great fault blocks, and most of the many nearly north-south Basin Ranges of Nevada and Utah are in reality recently tilted fault blocks.

Turning now to the Colorado Plateau, studies have shown that region to have been more or less periodically raised fully 20,000 feet since the beginning of Tertiary time, but because of profound erosion in the meantime its present altitude is only 6,000 to 9,000 feet. During late Tertiary time the land stood at a much lower level than to-day, so that, practically during the last period (Quaternary) of geologic time, the region has been elevated to itspresent position. As a direct result of this profound rejuvenation the Colorado River has had its erosive activity tremendously increased, and it has carved out the mightiest of all existing canyons—the Grand Canyon. The work of deepening and widening the canyon is still proceeding at a rapid geologic rate.

As we have learned, the Rocky Mountains and many of its subsidiary ranges were formed by folding and uplift of strata toward the close of the Mesozoic era (Cretaceous period). During much of Tertiary time the newly formed mountains had been considerably reduced by erosion. Then, late in the Tertiary period, much of the Rocky Mountain region, as well as much of the Great Plains area just east of the mountains, became rejuvenated by differential uplift without any notable folding of strata. We can tell that this general uplift amounted to at least several thousand feet because definite formations of relatively late Tertiary strata, originally horizontally deposited under inland bodies of water, gradually rise so that at the base of the Front Range of the Rockies they are fully 3,000 feet higher than they are 200 miles or more farther east. Thus, the original folding and faulting of the Rockies, Tertiary volcanic activity, late Tertiary rejuvenation, and subsequent erosion account for the present altitude and relief features of the great Rocky Mountain system.

Portions of the rejuvenated Great Plains region have been notably dissected by erosion since the late Tertiary, this being particularly true of the so-called “Bad Lands,” especially in parts of Wyoming and South Dakota, where mostly relatively soft Tertiary strata have been cut to pieces.

Turning our attention now to the eastern half of the continent we find that all, or nearly all, of it was more or less raised toward the close of the Tertiary period. Practically the whole Mississippi Valley east of the Great Plains, as well as much of the country to the north in Canada, was elevated some hundreds of feet and the streams have since the late Tertiary uplift (except where the land was ice-covered during the Ice Age) been at work sinking their channels below the newly upraised surface.

As already pointed out, the lowlands of the Atlantic and Gulf Coastal Plains were mostly submerged under the sea during early middle Tertiary time. By the close of the period they had emerged practically to their present positions, and they have been only moderately affected by erosion.

We have still to explain the existing topography or relief of a large and important part of eastern North America, including the whole of the Appalachian Mountains, Allegheny Plateau, Piedmont Plateau, New York, New England, and the Canadian region to the north. As a starting point in this discussion we should recall the fact that, after the great Appalachian Mountain Revolution toward the close of the Paleozoic era, the predominant geologic process which affected the region under consideration was erosion throughout the succeeding Mesozoic era. By about the close of the Mesozoic (Cretaceous period) the whole region, with some local exceptions, has been worn down to a comparatively smooth plain (peneplain) not far above sea level. Local exceptions were mainly in the New York and New England region as, for example, some of the higher parts of the Adirondackand White Mountains, Mount Monadnock in southern New Hampshire, and Mount Greylock in western Massachusetts. These and other masses rose rather conspicuously above the general level of the great plain of erosion commonly called the “Cretaceous peneplain” because it is believed to have been well developed by the close of that period.

The uplift of the vast Cretaceous peneplain about the beginning of the Cenozoic era (Tertiary period) was an event of prime importance in the recent geological history of eastern North America because it was literally the initial step in bringing about nearly all of the existing major relief features of the Appalachian-New York-New England-St. Lawrence region. The amount of uplift (unaccompanied by folding) of the peneplain was commonly from a few hundred to a few thousand feet with the greatest amount in general along the main trend of the Appalachians. The fact should be emphasized that nearly all the principal topographic features of the great upraised region have been produced by dissection (erosion) of the uplifted peneplain surface. Thus nearly all the valleys, small and large, including those of the St. Lawrence, Hudson, Mohawk, Connecticut, and Susquehanna, have been carved out by streams since the uplift of the great peneplain.

The streams which flowed upon the old low-lying peneplain surface meandered sluggishly over deep alluvial or flood-plain deposits, and their courses were little if any determined by the character and structure of the underlying rocks, because, with few exceptions, all rocks were worn down to the general plain level. The uplift of the peneplain, however, caused great revival of activity of erosive power bythe streams, the larger ones of which soon cut through the loose superficial alluvial deposits and then into the underlying bedrock. Thus the large, original streams had their courses well determined in the overlying deposits, and when the underlying rocks were reached the same courses had to be pursued entirely without reference to the underlying rock character and structure. Such streams are said to be “superimposed” because they have, so to speak, been let down upon and into the underlying rock masses. As Professor Berkey has well said: “The larger rivers, the great master streams, of the superimposed drainage system, in some cases were so efficient in the corrosion of their channels that the discovery of discordant structures (in the underlying rocks) has not been of sufficient influence to displace them, or reverse them, or even to shift them very far from their original direct course to the sea. They cut directly across mountain ridges because they flowed over the plain out of which these ridges have been carved, and because their own erosive and transporting power have exceeded those of any of their tributaries or neighbors.”

Fine examples of such superimposed streams which are now entirely out of harmony with the structure of regions through which they flow are the Susquehanna, Delaware, and Hudson. Thus the Susquehanna cuts across a whole succession of Appalachian ridges while, in accordance with the same explanation, the Delaware cuts through the Kittatiny range or ridge at the famous Delaware Water Gap. The ridges are explained as follows: while the great master streams were cutting deep trenches or channels in hard and soft rock alike, numerous side streams (tributaries) came intoexistence and naturally mostly developed along belts of weak, easily eroded rock parallel to geologic (folded) structure. Thus the Appalachian valleys have been, and are being, formed, while the ridges represent the more resistant rock formations which have more effectually stood out against erosion. The lower Hudson River flows at a considerable angle across folded formations above the Highlands, after which it passes though a deep gorge which it has cut into the hard granite and other rocks of the Highlands. The simple explanation is that the Hudson had its course determined upon the surface of the upraised Cretaceous peneplain, and that it has been able to keep that course in spite of discordant structure and character of the underlying rocks. In a similar manner we may readily account for the passage of the Connecticut River through a great gap in the Holyoke ridge or range of hard lava in western Massachusetts.

Before leaving this part of our discussion we shall briefly present some evidence showing that the New York-New England-St. Lawrence region at least must have been considerably higher shortly before the Ice Age (Quaternary period). An old channel of the Hudson River has been traced about 100 miles eastward beyond the present mouth of the river and it forms a distinct trench under the shallow sea in the continental shelf. Even in the Hudson Valley, many miles above New York City, the bedrock bottom of the river lies hundreds of feet (near West Point, 800 feet) below sea level. Obviously this submerged channel must have been cut when the land in the general vicinity of New York City was fully 1,000 feet higher than at present. That the land thus stood higher late in the Tertiary andpossibly early in the Quaternary periods is proved as follows: (1) because most of Tertiary time must have been needed for the river to erode such a deep valley after the initial uplift of the peneplain about the beginning of the period; and (2) because glacial deposits of Quaternary age filled the former channel to a considerable depth. The valleys of the coast of Maine, and the submerged lower St. Lawrence Valley (Gulf of St. Lawrence), in a similar way lead us to conclude that the region farther north was also notably higher just before the Ice Age.

In the eastern hemisphere early in the Tertiary period a great submergence set in and marine waters spread over much of western and southern Europe, northern Africa, and southern Asia. The sites of the Himalayas, Alps, Pyrenees, Apennines and other mountains were then mostly submerged. A very remarkable marine deposit, made up almost wholly of carbonate of lime shells of a single-celled animal called Nummulites, formed on the floor of this vastly expanded early Tertiary mediterranean. This rock attains a thickness of several thousand feet. It is doubtful if any other single formation made up almost entirely of the shells of but one species is at once so widespread and thick. In the Alps this remarkable marine deposit may be seen 10,000 feet above sea level, and in Tibet fully 20,000 feet. Much of the rock in the Egyptian pyramids was quarried from this formation.

Later in the Tertiary in Eurasia and Africa the marine waters gradually became very restricted, so that by the close of the period the relations of land and sea were not strikingly different from the present, although northwestern Europe, like northeasternNorth America, was notably higher just before the Ice Age than it is to-day.

Eurasia witnessed tremendous crustal disturbances during the middle and later Tertiary time when, due to intense folding and uplift of great zones, the Himalayas, Caucasus, Alps, Pyrenees, Apennines, and other great ranges were formed. The crustal disturbance was most remarkable in the region of the Alps, where the movement resulted in “elevating and folding the Tertiary and older strata into overturned, recumbent, and nearly horizontal folds, and pushing the southern or Lepontine Alps about sixty miles (over a low angle fault fracture) to the northward into the Helvetic region. Erosion has since carved up these overthrust sheets, leaving remnants lying on foundations which belong to a more northern portion of the ancient (early Tertiary) sea. Most noted of these residuals of overthrust masses is the Matterhorn, a mighty mountain without roots, a stranger in a foreign geologic environment.” (C. Schuchert.)

The last period of geological time—the Quaternary—was ushered in by the spreading of vast sheets of ice over much of northern North America and northern Europe, and this ranks among the most interesting and remarkable events of known geological time. On first thought the former existence of such vast ice sheets seems unbelievable, but the Ice Age occurred so short a time ago that the records of the event are perfectly clear and conclusive. The fact of this great Ice Age was discovered by Louis Agassiz in 1837, and fully announced before the British Scientific Association in 1840. For some years the idea was opposed, especially by advocates of the so-called iceberg theory. Now,however, no important event of earth history is more firmly established, and no student of the subject ever questions the fact of the Quaternary Ice Age.

Some of the proofs of the former presence of the great ice sheet are as follows: (1) polished and striated rock surfaces which are precisely like those produced by existing glaciers, and which could not possibly have been produced by any other agency; (2) glacial bowlders or “erratics” which are often somewhat rounded and scratched, and which have often been transported many miles from their parent rock ledges (Plate 20); (3) true glacial moraines, especially terminal moraines, like that which extends the full length of Long Island and marks the southernmost limit of the great ice sheet; and (4) the generally widespread distribution over most of the glaciated area of heterogeneous glacial débris, both unstratified and stratified, which is clearly transported material and typically rests upon the bedrock by sharp contact.

The best known existing great ice sheets are those of Greenland and Antarctica, especially the former, which covers about 500,000 square miles. This glacier is so large and deep that only an occasional high rocky mountain projects above its surface, and the ice is known to be slowly moving outward in all directions from the interior to the margins of Greenland. Along the margins, where melting is more rapid, some land is exposed, and often the ice flows out into the ocean where it breaks off to form large icebergs.

Back to Index Next