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.

In the eastern half of North America there is no record of accumulation of any marine strata whatever, because, as a result of the Appalachian Revolution, the land was brought well above sea level. There was, however, deposition of a remarkable series of nonmarine strata in several long, narrow, troughlike depressions whose trend was parallel to, and just east of, the main axis of the newly formed Appalachian Range. These troughs lay between the Appalachians and the very persistent old land mass called Appalachia which we have already described. The facts that these troughs are truly down-warps; that they so perfectly follow the trend of the Appalachian Mountain folds; and that the strata in them are of late Triassic Age, make it certain that they were formed by a great lateral pressure which must have been a continuation of the Appalachian Revolution. Thus the Appalachian Mountains continued to grow well into the Triassic period, and, while the Paleozoic strata were being folded, the surface of old Appalachia (including part of the Taconic Mountain region) was down-warped to form the troughs in which the late Triassic strata accumulated. One trough extended through the Connecticut Valley; another (the largest) from southeastern New York through northern New Jersey, southeastern Pennsylvania, Maryland, and into Virginia; while several smaller ones occurred in Virginia and North Carolina.

The down-warps or troughlike basins were very favorably situated for rapid accumulation of thick sedimentary deposits because of their position just between large, high land masses which were being vigorously eroded. The sediments derived from the erosion of the young Appalachians were especially abundant because of the vigorous wearing down of the newly formed high mountains. A thickness of from 5,000 to fully 15,000 feet of mostly red sandstones and shales accumulated in these down-warps, the character and great thickness of the strata strongly pointing to gradual down-warping as the deposition of the sediments went on. It is often stated that these strata were formed in estuaries, but, in the northern areas, at least from Massachusetts to Maryland, many of the layers show ripple marks, sun cracks, rain-drop pits, fossil plants, and fossil bones and tracks of land reptiles. Such strata may well have formed in very shallow water, such as river-flood plains or temporary lakes, where changing conditions frequently allowed the surface layers to lie exposed to the sun.

During the time of the accumulation of the late Triassic strata in the down-warp basins there was considerable igneous activity, as proved by the occurrence of sheets of igneous rock within the body of strata. In some cases true lava flows with cindery tops were forced out on the surface and then buried under later sediments, while in other cases the sheets of molten rock were forced up either between the strata or obliquely through them, thus proving their intrusive character. As a result of subsequent erosion, these very resistant lava masses often stand out conspicuously as relief features. Perhaps the most noteworthy example is the great layer of such intrusive igneous rock, part of which outcrops for seventy miles mostly as a bold cliff forming the famous Palisades of the Hudson, near New York City. During the process of cooling and solidification of the molten mass there was contraction which expressed itself by breaking the rock mass into great, crude, nearly vertical columns, and hence the origin of the name "Palisades." The cliff character of the outcrop is due to the fact that the lava is much more resistant to erosion than the sandstone above and below it. In the Connecticut Valley of Massachusetts a layer of lava several hundred feet thick boldly outcrops, forming the crest of the well-known Mount Tom-Mount Holyoke Range.

The close of the Triassic period was marked by enough uplift to leave the whole eastern two-thirds of the continent dry land undergoing erosion. The Triassic deposits of the Atlantic Coast are much broken up into large fault blocks, and this faulting probably took place as a result of the crustal disturbances toward the end of the period. In the west the Triassic conditions seem to have continued without much change into the next (or Jurassic) period.

During the Jurassic period the relations of land and water in North America were very simple. In the earlier Jurassic all was dry land except portions of the western fringe of the continent from southern California to Alaska, where marine strata 2,000 to 10,000 feet thick accumulated. Late in the period the conditions were the same, except for a long, narrow arm of the sea or mediterranean which extended from the Arctic Ocean southward across the site of the Rocky Mountains to Arizona. There is no evidence for the existence of anything like real mountains anywhere on the continent during the period.

As Cretaceous time went on the marine waters gradually spread until the whole Atlantic and Gulf coastal plain regions from Long Island, New York, to Mexico became submerged under marine water, and a wide arm of the sea, or great mediterranean, spread from Texas north to the mouth of the Mackenzie River. The Gulf of Mexico was thus directly connected with the Arctic Ocean. This great interior sea was nowhere connected with the Pacific Ocean, though portions of the Pacific border of the continent were submerged. This vast interior sea was not only the largest of any which reached well into the continent since the Mississippian period of the Paleozoic era, but it was the last body of marine water which ever extended well into the continent. It should be stated that the later Cretaceous was also a time of unusually widespread submergence of the continents, when most of southern Europe and southeastern Asia, as well as about one-half of both Africa and South America were submerged. Over much of the site of the Rocky Mountains during the late Cretaceous there were low lands receiving continental deposits, and extensive marshes supporting prolific vegetation were common. Much of this vegetable matter became buried, and has since been converted into workable coal.

The maximum thickness of strata accumulated during all of Cretaceous time over the Atlantic coastal plain area was about 1,700 feet; over the Gulf coastal plain region fully 7,500 feet; over the western interior 10,000 to 15,000; and over parts of the Pacific border 25,000 to 30,000 feet, as in California. The last-named figures are truly phenomenal, representing a thickness about equal to the total thickness of all the strata accumulated during the whole Paleozoic era (seven periods) and piled up in the Appalachian Mountain region. This great deposit of strata of mostly early Cretaceous Age is readily accounted for when we realize that these sediments, which accumulated in the marginal sea bottom, were derived from the very rapidly eroding, newly formed lofty Sierra Nevada Range.

Especially in Alabama and Texas the Cretaceous system is remarkable for its richness in chalk deposits. In Alabama a widespread formation of late Cretaceous Age, about 1,000 feet thick, contains much nearly pure white chalk, and in Texas a similarly constituted formation of early middle Cretaceous Age is from 1,000 to 5,000 feet thick. These chalk deposits consist almost wholly of carbonate of lime shells or very tiny single-celled animals which accumulated under exceptionally clear sea water which spread over those parts of Alabama and Texas where the chalk now occurs. Here again we have a bit of evidence supporting the fact of very long geologic time. Think of how long it must have taken for the tiny (even microscopic) shells to form a widespread layer of chalk nearly a mile thick!

The close of the Cretaceous period, or what is the same, the close of the Mesozoic era, was marked by some of the grandest crustal disturbances in the known history of the earth. In fact, it is not known that the western hemisphere was ever affected by more profound and widespread mountain-making disturbances than those which took place toward the close of the Mesozoic era, and continued into the succeeding Tertiary period. These disturbances were of three kinds: folding of strata, volcanic activity, and renewed uplift of old mountains without folding of the rocks. Greatest of all was the "Rocky Mountain Revolution," during which the thick strata, which accumulated during the Paleozoic and Mesozoic eras over the site of the Rockies, yielded to vigorous deformation when they were more or less folded and dislocated from Alaska to Central America. This was in truth the birth of the Rocky Mountains, although their existing altitude and configuration have, to a very considerable degree, resulted from later uplift and erosion. In the northern United States and southern Canada the Rocky Mountain strata, up to over 40,000 feet thick, were most severely folded and fractured, forming a range which quite certainly was fully 20,000 feet high. In this district a great thrust fault, hundreds of miles long, developed, and rocks as old as the Proterozoic were shoved at least seven miles, and probably as much as twenty miles, westward, over Cretaceous and other rocks much later than the Proterozoic. At the same time the Andes Mountains throughout South America were notably upraised and the rocks folded.

The second type of physical disturbance was volcanic activity which took place on a tremendous scale, and which appears to have started as a direct accompaniment of the Rocky Mountain Revolution. This igneous activity took place not only in the Rocky Mountains but also westward to and in the Sierra-Cascade Range, as well as in the mountains of western British Columbia and Alaska. This activity continued well into the succeeding Cenozoic era, and it is more fully considered in the next chapter.

The third type of crustal disturbance took place on a large scale when the Appalachian Mountains, which had been almost wholly planed away by erosion during Mesozoic time, were reelevated from 1,000 to 3,000 feet by an uplifting force not accompanied by folding. All or nearly all of New York and New England, as well as much of southeastern Canada, were similarly upraised at the same time. This notable uplift of so much of eastern North America is a matter of great importance because the major relief features of that area have been produced by erosion or dissection of the upraised surface since late Mesozoic or early Cenozoic time. In view of the fact that this work of erosion took place almost wholly during the Cenozoic era, it will be discussed in the next chapter.

In conclusion, brief mention may be made of the kind of climate of the Mesozoic era. As shown by the character and distribution of fossil plants and animals, the Mesozoic climate was in general mild and rather uniform over the earth, but with some distinction of climatic zones. Such distinction of climatic zones is unknown for the Paleozoic era, while it was notably less than at present.