Geology: The Science of the Earth's Crust

by William J. Miller

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 completeness and 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 in upturning 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 by [Figure 39].