Chapter 7

BEGINNING with the earliest Paleozoic, the legible records of events of earth history are far more abundant and less defaced than those of earlier times. Stratified rocks of the ordinary kinds greatly predominate over the igneous and metamorphic rocks, and the strata are in general far less disturbed than those of the Archeozoic and Proterozoic groups. From the earliest Paleozoic we have also the first abundant records (fossils) of the life of the earth, so that the ordinary methods of subdividing and determining the relative ages of the Paleozoic and later strata, as well as correlating the subdivisions (formations) in widely separated regions, can be used. From here on in our discussion of earth history we shall be able to trace the salient features of the changing outlines of the face of the earth, the coming and going of the seas over the lands, and the evolution of animals and plants with a considerable degree of definiteness and satisfaction.

First, we shall trace out, in the regular order of their occurrence, the main physical history events of Paleozoic time, leaving a consideration of the evolution of life for other chapters. Because of limitation of space, our attention will be almost wholly centered upon the continent of North America, but the reader should bear in mind that thegeneral principles and facts set forth apply with about equal force to most other continents. In Europe the wonderful records of Paleozoic history are found in strata, whose estimated maximum thickness is about 100,000 feet! It must not be thought, however, that all these strata are piled up in a single locality, but the figure does actually represent the sum total of the greatest thickness of the many subdivisions (formations) of the Paleozoic rocks in different portions of the continent. In North America the maximum thickness of all Paleozoic rocks seems to be no less than 50,000 feet. More than 25,000 feet of strata may actually be observed piled layer upon layer in the highly folded and deeply eroded central Appalachian Mountains. The great thickness of the strata, combined with the facts that the fossils show that many marvelous, mostly progressive, changes took place among living things, that seas came and went repeatedly over many parts of the continent, and that great changes took place in the configuration of the land, force us to conclude that Paleozoic time must have lasted for many millions of years.

Just before the opening of the Paleozoic era practically all of North America appears to have been dry land, which had undergone so much erosion that it was low and far less rugged in relief than at present. This we know, because the rather widespread early Paleozoic (Cambrian) strata almost everywhere rest upon deeply eroded rocks of either Archeozoic or Proterozoic age. Considering both the time involved and the wide area affected, we have no record of anything like such a profound erosion interval since the beginning of the Paleozoic era. It seems that the constructive or upbuildingforces within the earth were then remarkably quiescent, while the destructive forces (erosion) were almost unhampered in their work of cutting down the land.

Have we any definite idea of the relations of land and water in North America during the first or Cambrian period of the Paleozoic era? In the affirmative answer to this question, certain principles will be brought out which the reader should keep in mind as we trace out the succeeding great physical changes in the history of North America. It should, however, be remembered that, in the brief space at our disposal, only the most general, or the most significant localized, physical changes in the long and intricate known history of the continent since the opening of the Paleozoic era can be brought out.

In early Cambrian time a narrow arm of the sea (like a strait) extended from the Gulf of St. Lawrence southward across eastern New York and over the site of the present Appalachian Mountains connecting with the Gulf of Mexico on the south. On the west, a much larger and broader arm of the sea (like a mediterranean) extended from Alaska southward over the site of the Rocky Mountains of Canada and across the sites of the Columbia Plateau to Great Basin of the western United States. All the rest of the continent was land, apparently almost or wholly devoid of high mountains.

By what process of reasoning do we conclude that arms of the early Cambrian sea reached across eastern and western North America? First, wherever marine strata of definitely determined early Cambrian age now occur, the early Cambrian sea must have existed because those strata were obviously deposited in that sea. Second, to those areas we mustadd others from which it can be demonstrated that early Cambrian marine strata have been removed by erosion. Enough field work along these lines has been done in North America to render it practically certain that the relations of land and water during early Cambrian time were essentially as above outlined.

Fig. 33.—Map showing the relations of land and water in North America during early Cambrian time, at least 25,000,000 years ago. Lined areas represent land. (Principal data from a map by Willis published in the Journal of Geology.)

Fig. 34.—Map showing the relations of land and water in North America during Middle Ordovician time. Lined areas represent land. (Principal data from a map published by Willis in the Journal of Geology.)

As Cambrian time went on, the marine waters gradually spread from south to north across most of the Mississippi Valley area, causing the eastern and western arms of the sea to be connected, thus forming an interior continental sea. Otherwise the relations of land and water were much as in early Cambrian time. We know that the sea transgressed northward across the Mississippi Valley district because, on the south, the whole Cambrian system of strata (lower, middle, and upper) is present, while, farther north, only middle and upper Cambrian are present, and, farthest north, only upper Cambrian strata occur. This progressive northward overlap of younger and younger (later) Cambrian strataupon the old rock floor proves, that the Cambrian sea steadily spread farther and farther northward over the Mississippi Valley area. That this spreading sea was shallow is amply demonstrated by the deposits it left, such as shales, conglomerates (i.e., consolidated gravels) and sandstones, often ripple-marked. The Cambrian strata of North America vary in thickness from less than 1,000 feet to about 12,000 feet.

In the Mississippi Valley the Cambrian strata are unaltered and almost undisturbed from their original horizontal position. In the Appalachian Mountains of the east, and the Rocky Mountains of the west, the strata are commonly notably folded and faulted. In some places, as in western New England, the strata have been notably altered (metamorphosed).

The best estimates for the duration of the Cambrian period range from 2,000,000 to 3,000,000 years. It is a remarkable fact that, during this great lapse of time, North America was unaffected by any great physical disturbances such as mountain making, emergence of large tracts of land, or igneous activity. The one great physical event of the Cambrian was the gradual submergence of a considerable portion of the continent.

That the climate of the earliest Cambrian was at least locally favorable for the existence of glaciers, is proved by the occurrences of true glacial deposits in rocks of that age in China, Norway, and Australia. It is a remarkable fact that the glacial materials of China occur along the Yangtse River, thus demonstrating that conditions for glaciers then existed at a latitude as far south as New Orleans. These evidences of glaciation directly refute the oldidea, based upon the nebular hypothesis, that the climate of the Paleozoic was distinctly warmer than now. The glacial evidence, added to our knowledge of the character and world-wide distribution of many identical species of animals, leads us to conclude that early Paleozoic climate was not essentially different from that of very recent geologic time, but that the climate was then much more uniform than at present.

During the second or Ordovician period of the Paleozoic era, the progressive submergence of Cambrian time continued until a climax was reached toward the middle of the period when fully four-fifths of the continent was submerged under shallow sea water. Since middle Ordovician marine strata are more widespread than the rocks of any succeeding age, we can be reasonably sure that so much of the continent was never again covered by the sea. In fact, so far as the records have been interpreted, this came nearest to being a universal flood in the whole known history of the continent. By the very character of the rocks deposited (seldom over a few thousand feet thick), we can be sure that the middle Ordovician continental sea was everywhere far shallower than the great ocean abysses of to-day. Because the lands were so low and restricted, relatively little land-derived sediment washed into the sea. But the shallow sea water was inhabited by millions of animals, the shells of many of which slowly accumulated to build up the thick bodies of limestone strata (Plate 14) which constitute the main bulk of rock of early and middle Ordovician age. The famous Trenton limestone, named from a locality in central New York, with its great abundance of fossils, was formed mostly by theaccumulation of shells of animals during middle Ordovician time.

Later in the Ordovician there was a considerable shift in level between land and water causing a withdrawal of much of the widespread sea. As a result of the generally more elevated lands, erosion proceeded more vigorously, and sands and muds were more abundantly deposited in the restricted sea, these sediments having consolidated to form the shales and sandstones which predominate among the upper Ordovician rocks.

A principle above briefly explained in the discussion of the Cambrian may be reemphasized here. It is as follows: In making a map to show the relations of land and water, say during middle Ordovician time, the geologist is by no means dependent only upon actual surface exposures of middle Ordovician strata. Such exposures fall far short of giving an adequate conception of the former or even present real extent of such strata. In many places originally present Ordovician strata have been removed by erosion. An excellent case in point is the Adirondack region of northern New York. On the west side of the Adirondacks a great pile of marine Ordovician strata 1,500 feet thick end abruptly on the gently sloping flank of the mountains, thus clearly proving that the strata formerly extended at least twenty to thirty miles eastward. Again, in the southern Adirondacks a small area of very typical marine middle Ordovician strata lies fully fifteen miles from the general area of such rocks to the south. This small body of rock is very clearly only an erosion remnant of a general sheet of middle Ordovician rock which once covered the whole intervening district. In many other regions themiddle Ordovician strata are definitely known to be concealed under cover of later rocks, as in the Mississippi Valley, where the actual surface exposures constitute only a fraction of the middle Ordovician strata which underlie nearly all the valley, as proved by deep well drillings, study of the scattering outcrops, etc. In still other places, middle Ordovician strata, associated with other rocks, are highly folded, as in the Appalachians, where such strata outcrop in only narrow belts following the trend of the folds. In short, then, wherever it can be proved that middle Ordovician marine strata are visible at the surface, or are concealed under other rocks, or were once present, we can be sure that the middle Ordovician sea existed. Exactly this principle applies to any subdivision of geologic time.

Fig. 35.—Structure section showing rocks representing three geologic eras separated by millions of years of time. Length of section 12 miles, vertical scale much exaggerated. At the bottom are Archeozoic (Precambric) rocks and resting upon them on the left are early Paleozoic strata 1,500 feet thick. A glacial lake deposit of late Cenozoic age lies on the Archeozoic rock toward the right. It is evident that the Paleozoic strata formerly extended much farther eastward. (By the author as published in a New York State Museum Bulletin.)

The Ordovician period closed with a great mountain-making disturbance in eastern North America,and at the same time all, or nearly all, of the continent was land. Throughout most of the Cambrian and Ordovician periods, the strata accumulated to a thickness of thousands of feet in the marine waters which spread over the eastern border of New York, the sites of the Green Mountains of Vermont, the Berkshire Hills of Massachusetts, and southward at least as far as Virginia, over the area of the Piedmont Plateau. At, or toward the close of the Ordovician period, a great compressive force in the earth’s crust was brought to bear upon the mass of strata and they were tilted, highly folded, and raised above sea level into a great mountain range known to geologists as the Taconic Range. It is quite the rule throughout this region of Taconic disturbance to find the strata either on edge or making high angles with the plane of the horizon. Many of the folds were actually overturned, and in some cases notable thrust faults developed, that is, the upper strata broke across and great masses were shoved over each other. These facts all go to show that the mountain-making compressive force applied to the region was of rather an extreme type. Since the origin of the Taconic Range a tremendous amount of erosion has taken place, so that literally only the roots of the range are now exposed in the Green Mountains, Berkshire Hills, Highlands-of-the-Hudson, and the northern Piedmont Plateau.

How do we know that the Taconic disturbance took place toward the close of the Ordovician period? By way of answer to this question two facts need to be considered. First, relatively late (or young) Ordovician strata are involved with the folds, thus proving that the folds formed after those late Ordovician sediments were deposited. Second, undisturbedstrata formed during the middle of the next (Silurian) period, rest upon the eroded edges of the folds, which proves that the folds must have developed well before middle Silurian time because the only time they were subjected to erosion must have been during early Silurian time.

Fig. 36.—Structure section showing profile and underground relations of the rocks across part of the Highlands-of-the-Hudson region in southeastern New York. Length of section, sixteen miles. The rocks are mostly of Prepaleozoic Age, but with belts of highly infolded early Paleozoic strata toward the middle right. (After Berkey, New York Museum Report.)

Mention should also be made of the profound metamorphism (alteration) of the Cambrian and Ordovician strata along the main axis of the range, where the intense compression, aided by heat and moisture, caused the deeply buried portions of the strata to become plastic, and hence they became more or less foliated (cleavable) and crystallized into various metamorphic rock types, the limestone having changed to marble, the shale to slate or schist, and the sandstone to quartzite. Thus we explain the rocks of the extensive marble quarries of Vermont and western Massachusetts, the slate quarries of central eastern New York, andthe Berkshire schist of the Berkshire Hills of Massachusetts.

One of the grandest and most significant of all the profound geological processes is the birth and history of a great folded mountain range. Since the Taconic Range affords us such an excellent example of a large-scale, well-understood folded range of great antiquity we may do well to consider it in the light of certain other broad relationships. The great compressive force which folded and upraised the Taconic Mountains did not accomplish its work rapidly in the ordinary human history sense of the word. The force was slowly and irresistibly applied, and the strata well below the surface were gradually bulged or folded, or fractured where near the surface, the length of time required for the operation having been, at the very least calculation, some hundreds of thousands of years, and more than likely a million years or more. Such a length of time is, however, so short compared with all known earth history, that we are accustomed to refer to the formation of such a mountain range as simply an event of geological history.

Even before such a range attains its maximum height a very considerable amount of erosion has already taken place. When the first fold appears above sea level, erosion begins its work and continues with increasing vigor as the mountain masses get higher and higher. Thus we have warfare between two great natural processes—the building up and the tearing down. After a time the building-up process wanes and then ceases, while the tearing-down process (erosion) continues either until the whole range has been completely worn down or until some rejuvenating force causes a renewed uplift.Here is an example of one of the remarkable procedures of nature. After millions of years of work causing the deposition of thousands of feet of strata, piled layer upon layer on the sea floor, a force of lateral pressure is brought to bear and a mountain range is literally born out of the sea. No sooner is the range well formed than the destructive processes (erosion) unceasingly set to work to destroy this marvelous work. But the sediments derived from the wear of the range are carried into the nearest ocean again to accumulate and, perchance, after long ages, to be raised into another range; and so the process may be often repeated. From this we learn that the mountain ranges of the earth are by no means all of the same age. The original Adirondacks were formed long before the Taconics, which originated millions of years before the Appalachians, these latter having been folded up long before the Sierras. The Rockies, followed by the Coast Ranges, are each younger than the Sierras as regards their original folding and uplift. Among foreign countries special mention should be made of the British Isles, where Ordovician strata thousands of feet thick were, late in the period, notably folded and upraised, the crustal disturbance having been accompanied by great intrusions of molten rocks and vast outpourings of lavas, so that this region ranks among the greatest of the ancient volcanic areas of Europe.

Fig. 37.—Map showing the general relations of land and water in North America during middle and late middle Devonian time fully 15,000,000 years ago. (After Willis, courtesy of the Journal of Geology.)

We shall now turn our attention to a very brief consideration of the salient points in the physical history of North America during the next great period (Silurian) of the Paleozoic era. As a result of the physical disturbance late in the Ordovician the great interior sea was largely or wholly expelled from the continent, and this was essentially the condition of the continent at the beginning of the Silurian. But this condition was of short duration, for early in the Silurian the sea again began to spread, gradually increasing in extent to a climax in about the middle of the period. At this time the famous and extensive Niagara limestone, so named from the rock at the crest of Niagara Falls, was deposited. Except for the newly formed Taconic Range, standing out as a bold topographic feature along the middle Atlantic Coast, and asomewhat wider extent of land, the condition of the continent during middle Silurian time was very similar to that of middle Ordovician time.

Fig. 38.—Map showing the general relations of land and water in North America during middle Mississippian time. (After Willis, courtesy of the Journal of Geology.)

Soon after mid-Silurian time the seas became greatly restricted almost to disappearance as such. In the eastern United States and southeastern Canada strata of that particular age are found only in parts of Ontario, New York, Ohio, Michigan, and from Pennsylvania southward to West Virginia, where they are characterized by red shales and sandstones, and salt and gypsum deposits. Suchmaterials containing few fossils very clearly indicate deposition in either extensive lagoons or more or less cut-off arms of the sea under arid climate conditions rather than in ordinary marine water.

Still later in the Silurian the interior seas were partially restored, as shown by the fact that true marine strata corresponding to that age not only cover the salt and gypsum deposits, but are notably more extensive than they. About the close of the Silurian period almost all of the continent was dry land.

Unlike the Ordovician period, the Silurian closed without any mountain-making disturbance or great uplift of land. The Silurian period, like the preceding Ordovician and Cambrian, seems to have been free from any more than slight igneous activity as, for example, in Maine and New Brunswick. The total thickness of Silurian strata in North America is seldom more than a few thousand feet.

The salient features of the physical history of the next, or Devonian period, are much like those of the preceding Silurian. At the beginning of the Devonian almost all of the continent was dry land, but soon a long, narrow arm of the sea extended across the eastern side of the continent from the Gulf of St. Lawrence southward through western New England, southeastern New York and throughout the Appalachian district, thus reminding us of the long, narrow sound which occupied almost exactly the same territory during the early part of the Cambrian period. In the west the only water was a small embayment reaching across southern California into Nevada. By middle Devonian time these water areas had considerably expanded. During relatively late Devonian time the sea was so expandedas to cover much of the Mississippi Valley area, the Appalachian Mountains and St. Lawrence Valley areas, and most of the site of the Rocky Mountains, except for an island of considerable size reaching from New Mexico through Wyoming. The main lands were most of northeastern North America, a large land area extending from Florida to Nova Scotia, and a large area on the western side of the continent from California to Alaska.

A remarkable formation of late Devonian Age should be briefly described. In southeastern New York and the northern Appalachian region there was a tremendous accumulation of sediments which have consolidated into sandstone, together with some shale and conglomerate. This so-called “Catskill” formation is from 1,500 to 8,000 feet thick and is well shown as the main body of rock in the Catskill Mountains. It is largely a shallow-water deposit of essentially nonmarine origin, as proved by coarseness of material, ripple marks, and nonmarine fossils. All evidence points to the origin of this remarkable formation as a great delta deposit built out into the shallow interior sea. Notable thinning toward the west, with increasing fineness of grain of material, shows that the sediment came from the east, no doubt carried by a large river from the small continental land mass (called “Appalachia”) on the eastern side of North America.

The maximum thickness of the North American Devonian seems to be about 15,000 feet in the northern Appalachian region, but elsewhere it generally ranges from 1,000 to 4,000 feet thick. In North America the subdivisions of the Devonian strata of New York are taken as a standard for comparison, both because of the wonderful completenessand almost undisturbed character of the rocks there, and because they have been so carefully studied. The Devonian system is there fully 4,000 feet thick, with scarcely a minor subdivision missing, and it covers a wide area (one-third of the State) with many excellent outcrops. There was practically uninterrupted deposition of Devonian strata in southern New York. It is doubtful if there is greater refinement of knowledge regarding the Devonian or any other Paleozoic system of strata anywhere else in North America.

During middle to late Devonian time the region from southern New England to Nova Scotia and the St. Lawrence Valley was notably disturbed by earth movements, the lands having been considerably elevated and the rocks more or less folded. The great delta deposit of late Devonian time, already described as being thousands of feet thick in New York and Pennsylvania, was formed by one or more streams which carved much sediment from the newly upraised lands. Accompanying the uplift and folding of the rocks considerable masses of molten granite were forced into the earth’s crust and some molten rock was forced to the surface, producing volcanoes. Much of the granite may now be seen at the surface in various portions of the region, while deeply eroded volcanoes occur near the city of Montreal.

Except for the disturbance of the region from New England to the St. Lawrence, the Devonian period seems to have closed rather quietly, with fairly widespread sea water over the land as already outlined. This is proved by the fact that the early strata of the next period mostly rest in regular order upon the undisturbed late Devonian strata.

For many years the term “Carboniferous” period was used to designate a single period of geologic time which, in America at least, is now divided into two periods—the Mississippian and Pennsylvanian—corresponding, respectively, to the earliest and latest Carboniferous. In regard to the relations of land and water during the Mississippian period, the general statement may be made that the sea, already fairly extensive in the late Devonian, continued to spread until during the second half of the Mississippian, when most of the United States west of the eastern border of the Appalachians (except the Pacific Northwest), and also the Rocky Mountain region through Canada, were submerged.

A significant physical change marked the close of the Mississippian. This was the withdrawal of sea water from nearly all of the continent, the emergence of the land having been generally sufficient to allow considerable erosion. The fact that the Mississippian and the next, or Pennsylvanian, strata are separated by the most extensive distinct erosion surface in the whole Paleozoic group of rocks is the chief reason for considering those two sets of strata to have formed during separate periods of geologic time.

In eastern North America the Mississippian strata vary in thickness from a few hundred feet to a maximum of about 5,000 feet in eastern Pennsylvania. In the West, where the thickness is commonly several thousand feet, limestone greatly predominates. There appears to have been vigorous volcanic activity during the period from northern California to Alaska.

Certain profound crustal disturbances marked the close of the period in western Europe, resulting inupturning and folding of rocks during the process of mountain forming from Ireland to Germany, and from Bohemia to southern France. Abundant intrusions and extrusions of molten rocks accompanied the disturbances.

We turn next to a consideration of the Pennsylvanian period, which is of very special interest, because within the rocks of that age in North America, Europe, and China occur the greatest known coal deposits. The period opened with almost all of North America dry land undergoing more or less erosion. Early in Pennsylvanian time marine water began to overspread the western side of the continent, especially most of the western two-thirds of the area of the United States, where strata thousands of feet thick piled up. The sea was most widespread before the middle of the period, when the relations of land and water were about as shown byFigure 39.

Over the site of the Appalachians and most of the eastern half of the Mississippi Valley area the land either stood near sea level and was often swampy or marshy, or at other times it was a little below sea level, allowing tidewater to overspread the area. Such conditions alternated repeatedly, usually more or less locally, over different parts of the districts in which the great coal mines of the east are located. Under such conditions strata from 1,000 to 8,000 feet thick accumulated. Remarkable physical geography of this kind resulted in the growth and accumulation of vast quantities of vegetable matter which has changed into the world’s greatest coal beds. Similar conditions prevailed over parts of Nova Scotia, New Brunswick, and Rhode Island, where strata fully 13,000 feet thick accumulated.

“Perhaps the most perfect resemblance to coal-forming condition is that now found on such coastal plains as that of southern Florida and the Dismal Swamps of Virginia and North Carolina. Both of these areas are very level, though with slight depressions in which there is either standing water or swamp condition. In both regions there is such general interference with free drainage that there are extensive areas of swamp, and in both there are beds of vegetable accumulations. In each of these areas there is a general absence of sediment and therefore a marked variety of vegetable deposit. If either of these areas were submerged beneath the sea, the vegetable remains would be buried and a further step made toward the formation of a coal bed. Reelevation, making a coastal plain, would permit the accumulation of another coal bed above the first, and this process might be continued again and again.” (H. Ries.) But it is not necessary to assume repeated oscillations of a swamp area up and down as the only way of accounting for a succession of coal beds one above another in a given region, because a general, but intermittent, subsidence, with possibly some upward movements, would occasionally cause the prolific plant life of a swamp to be killed, after which sediment would deposit over the site. Shoaling of water by accumulation of sediment would permit the development of more swamp plant life.

Fig. 39.—Map showing the general relations of land and water, including the great coal-plant swamp areas (vertical lines), in North America during the Pennsylvanian period at least 10,000,000 years ago. Lined areas represent land. (After Willis, courtesy of the Journal of Geology.)

In most coal-mining districts there are at least several coal beds, one above another. In Illinois there are nine; in Pennsylvania at least twenty; in Alabama, thirty-five; and in Nova Scotia seventy-six, but not all are important commercially. Each coal bed in such a region represents a swamp which existed in Pennsylvanian time at least ten or twelve million years ago, and in which there grew a luxuriant vegetation. Many individual swamps of that time were of wide extent. The famous Pittsburgh bituminous coal bed represents probably the largest one of all. It extends from western Pennsylvania into parts of Ohio and West Virginia over an area of fully 15,000 square miles. More than 6,000 square miles of it are being worked and the coal bedaverages seven feet in thickness over an area of 2,000 square miles. Among the various anthracite coal beds of the same age in eastern Pennsylvania the Mammoth bed is exceptionally thick, reaching a maximum of fifty feet or more.

In order that the reader may not gain the impression that coal beds make up a very considerable bulk of the strata in coal-mining regions, we should state that, on the average, coal actually constitutes less than 2 per cent of the containing strata.

Some idea of the tremendous length of the geologic ages may be gained by a consideration of the time which must reasonably be allowed for the accumulation of so many coal beds and their containing strata. It has been estimated that a luxuriant growth of vegetation would produce 100 tons of dried organic matter per hundred years. Compressed to the specific gravity of coal (1.4) this would form a layer less than two-thirds of an inch deep on an acre. During the chemical alteration of vegetable matter to coal about four-fifths of the organic matter disappears in the form of gases. On this basis, then, it would take about 10,000 years to accumulate the vegetable matter represented in a coal bed one foot thick. When we realize that the total thickness of the coal beds of the Pennsylvanian system of strata in the great mining regions is commonly from 100 to 250 feet, we conclude that the time they represent is from 1,000,000 to 2,500,000 years. It seems most reasonable that the time necessary for the deposition of the containing strata must have been at least as long. It is, therefore, a fair conclusion that the Pennsylvanian period lasted from 2,000,000 to 5,000,000 years.

That the climate of the great Coal Age was warm (not tropical), very moist, and uniform, is borne out by such facts as the following, according to D. White: The succulent nature of the plants with their spongy leaves indicates prolific growth in moist, mild climate; lack of yearly rings of growth points to lack of distinct seasons; as in the case of many existing plants the aerial roots signify a warm, moist climate; plants of to-day nearest like the coal plants thrive best in warm, moist regions; vegetable matter at present accumulates best in temperate rather than tropical climates, because there decay is not so rapid; and the remarkable uniformity of climate over the earth is clearly indicated by finding fossil plants of almost or exactly identical types in rocks of Pennsylvanian Age from the Polar regions to the Tropics. The more remarkable plants of the great Coal Age time are described in the chapter on the evolution of plants (Plate 15).

During the last (Permian) period of the Paleozoic era the marine waters of the west, and the alternating shallow tidewater, swamps, and near sea level lands of the east gradually gave way to dry lands, so that by the close of the period marine water covered only a small part of the Southwest from Oklahoma across central Texas to southern California and northwestern Mexico, where strata as much as several thousand feet thick formed. In the middle western part of the area of the United States, especially from northern Texas to Nebraska and Wyoming, the climate was arid and red strata (so-called “Red Beds”), salt, and gypsum were extensively deposited on land and in great salt lakes or more or less cut-off arms of the sea. Strata commonly from 2,000 to 7,000 feet thick were there deposited.Similar conditions prevailed in parts of Nova Scotia, New Brunswick, and Newfoundland, where strata 8,000 feet thick accumulated. Over the site of most of the Appalachians the coal swamp conditions, with local sea incursions, continued from the preceding period, as shown by the character of the strata (1,000 feet thick) containing some coal.

Vigorous volcanic activity which, as already mentioned, began in the Mississippian period from northern California to Alaska continued not only through the Pennsylvanian and Permian but also into the early Mesozoic era, as shown by the great quantities of volcanic materials associated with rocks of those ages.

The Permian presents a puzzling combination of climatic conditions which causes it to stand out in marked contrast against the generally mild and uniform climates of nearly all of preceding Paleozoic time. Most remarkable of all are the records of a great Ice Age during early Permian time. One surprising fact is the widespread distribution of the glacial deposits in both the north and south temperate zones, and even well within the torrid zone. They are perhaps most extensive and best known in Australia, South Africa, India, and Brazil. Glacial deposits almost certainly of the same age on smaller scales occur in eastern Massachusetts, southern England, eastern Russia and the Caucasus region. Although the areas occupied by the Permian glaciers, which in many cases must have been extensive ice sheets, cannot be accurately delimited, it is, nevertheless, quite certain that the ice was notably more extensively developed than it was during the great “Ice Age” of late (Quaternary) geologic time. Another surprising fact is that certain of theglaciers must have come down to, or nearly to, sea level, as shown by the direct association of marine strata with glacial deposits. Thus, in southern Australia at least eight beds of glacial materials (some of them 100 to 200 feet thick) occur within true marine strata 2,000 feet thick. A third remarkable fact is that the Permian Ice Age, like the Quaternary Ice Age, had interglacial epochs of relatively mild climate, as proved by the occurrences of beds of coal between certain of the layers of glacial materials in Australia, South Africa, and Brazil.

During much of Permian time the climate was arid over large areas as, for example, much of the western interior of the United States, from Ireland to central Germany, and in eastern Russia, as proved by great deposits of salt, gypsum, and red sediments. During late Permian time the greatest salt beds in the world were deposited in northern Germany, a well near Berlin having penetrated a practically solid body of salt associated with certain potash and magnesia salts to a depth of about 4,000 feet without reaching the bottom.

The occurrence of some coal beds, especially in the earlier Permian rocks shows that, temporarily at least, climatic conditions must have favored luxurious growths of coal-forming plants in South Africa, Brazil, Australia, and our own Appalachian district.

From the above facts we see that the Permian represents a remarkable combination of very extensive glaciation, widespread aridity, and warmth and moisture favorable to prolific plant growth all in a single period of geologic time.

The Permian period, and, therefore, the great Paleozoic era, was brought to a close by one of themost profound physical disturbances in the known history of North America. This has been called the Appalachian Revolution because at that time the Appalachian Mountain range was born out of the sea by folding and upheaval of the strata. In fact, “the Appalachian Revolution was one of the most critical periods in the history of the earth, and may have been the greatest of them all in its results.” (C. Schuchert). Mountains were brought forth in all the continents, including Australia. All of the mountains which were formed late in the Paleozoic have since been profoundly affected by erosion, and the only ones (e.g., Appalachians) which now show considerable altitudes are those which have been rejuvenated by relatively (geologically) recent earth movements.

We shall now turn our attention to the origin of the Appalachian Range. All through the vast time (probably fully 20,000,000 years) of the Paleozoic era a large land mass was remarkably persistent along the eastern side of North America. This land, which has been called “Appalachia,” had its western boundary approximately along the eastern border of the sites of the Appalachian Range and the western part of New England. It extended east of the present coast line at least to the border of the continental shelf from 100 to 200 miles out. Concerning the actual altitude and topography of Appalachia we know little or nothing, but the tremendous quantities of sediment derived from its erosion show that it was high enough during nearly all of its history to undergo vigorous erosion.

Barring certain minor oscillations of level, the region just west of Appalachia was mostly occupied by sea water throughout much of Paleozoic time,and sediments derived from the erosion of Appalachia were laid down layer upon layer as strata upon that sea bottom. In general, the coarsest and greatest thickness of sediments accumulated relatively near the land, while finer materials, in thinner sheets, deposited well out over much of the eastern Mississippi Valley area in the shallow seas which were there so commonly present. By actual measurement we know that the thickness of strata deposited over the site of the Appalachians was at least 25,000 feet. Since these latter strata are mostly of comparatively shallow sea-water origin, as proved by coarseness of grain of material, ripple marks, fossil coral reefs, etc., we are forced to conclude that this marginal sea bottom gradually sank while the process of sedimentation was in progress. Otherwise we cannot possibly explain the great pile of strata of shallow water origin. The very weight of accumulating strata may either have aided or actually caused the sinking of the long, relatively narrow trough.

Finally, toward the close of the Paleozoic era, sinking of the marginal sea floor and deposition of sediments gave way to a yielding of the earth’s crust by a great force of lateral compression, causing the strata to be thrown into folds well below the surface and more or less fractured in their upper portion. Thus, along the eastern side of the site of the great interior Paleozoic sea, the Appalachian Mountains rose out of what for millions of years had been a long, narrow, sinking sea floor. There was more or less folding from the Gulf of St. Lawrence to central Alabama.Figure 24diagrammatically represents the principal stages in the history of the Appalachian Range.

While the most pronounced earth disturbance occurred through the long Appalachian belt, the whole eastern side of the continent was profoundly affected. Thus the Mississippi Valley area east of the Great Plains was considerably upraised never again to be submerged except along the Gulf Coast, and an eastern interior sea has never since overspread the region which was repeatedly sea-covered during Paleozoic time.

CHAPTER XV

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.

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