Geology: The Science of the Earth's Crust

by William J. Miller

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 the glaciers 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 the most 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.