Geology: The Science of the Earth's Crust

by William J. Miller

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 strata upon 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 old idea, 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 the accumulation 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 the middle 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.)