Earth’s sedimentary crust

Source Areas

Great study has been given to the problem of locating highland sources of sediments now contained both in mountain ranges and in plains regions. Paleogeographical maps have been laboriously drawn for all geologic ages. Many conclusions are necessarily purely conjectural, involving the assumed existence of former high land masses for which no evidence is apparent.

This is admitted in a statement of Kuenan (p. 126) that “In constructing paleogeographical maps the necessity is frequently encountered of assuming former regions of denudation outside the present limits of the continental blocks.” (Italics added.)

Nevin says (Principles of Structural Geology, p. 288):

A study of mountain ranges bordering coast lines has shown that the sediments which make up these folded mountains must have been derived from large land masses that existed where the ocean now is. The source of the sediments which compose the Appalachians was from the east; and the volume deposited indicates the former presence of high land considerably beyond the edge of the present continental shelf. Likewise the source of sediments which form the Andes was from the west, right where the deep Pacific Ocean now exists.

H. G. Richards, in Record of the Rocks (1953), says (p. 113) that the sediments which filled the “Appalachian Geosyncline” were “carried down from highlands on the two sides of the geosyncline.” He says that the land to the southeast of the geosyncline “was high and mountainous” and “may, at times, have extended as much as 100 miles east of our present shore line.”

Those who accept the foregoing assumptions conclude that after former off-shore highlands were denuded to supply sediments for the geosynclines of present coastal mountains, they subsided to become and remain parts of oceanic floors. Yet, strangely, Nevin says (op. cit., p. 287): “Once given a continental block, there appears to be no reasonable way by which the density may be permanently changed the amount necessary to sink it and keep it on the ocean floor.” Does he contend that whereas an entire platform could not sink, substantial coastal segments extending “considerably beyond” continental shelves could do so?

Speaking of rock strata in Glacier National Park, Daly says that lands east and west of the Park supplied sediments for those rocks. Dr. Dyson states that the geosynclinal trough which subsequently was uplifted to form the Rocky Mountains was filled with sediments derived “from adjacent lands.” If what

Messrs. Daly and Dyson say is true, how do they explain the fact that rocks of the very same ages as those in the Rockies are found westwardly clear to the Pacific and eastwardly clear to the Atlantic?

Colorado-Arizona-Utah Plateau

The Grand Canyon, Zion and Bryce Canyon parks together contain strata of every geological period from Pre-Cambrian to Pleistocene, with the exception only of Ordovician and Silurian. At least the latter two are not exposed in the Grand Canyon. Except for discontinuous and very thin Devonian strata, late Cambrian rocks in the canyon are directly overlain by early Carboniferous. However, it is conceivable that the missing strata may underlie other areas of the plateau; hence their absence in the Grand Canyon is not considered proof that they were not deposited because the region was uplifted above sea level during those periods. Rather, it is believed by many geologists that the missing strata were eroded during the long lapse of time between the Cambrian and the Devonian.

Apparendy the region between the Rockies and the Sierras must have uninterruptedly received sedimentation throughout geological time and, if the orthodox theory of sedimentation is correct, the region continuously remained submerged below sea level. From data so far obtained in exposures and from well drilling, there can be little doubt that like sedimentation continued also east of the Rockies. Unfortunately, exposures even remotely approaching those in the Grand Canyon do not exist in eastern areas to make determination definite. That strata of a given period are missing in regional exposures eastwardly does not necessarily prove that they did not once exist there. Certainly, it is just as reasonable to believe the missing strata once existed but were subsequently removed by ablation, as it is to conclude and insist they never did exist because, during that particular period, the region stood above sea level.

Rocky Mountain Sediments

Did then sediments for the Rockies, which Daly and Dyson say came from the east, originate far away in the region of the Canadian Shield? Those coming from the west could have come neither from the basin to the west, nor from Pacific Coast ranges, for both the basin and those coastal mountains contain rocks of the same ages as some of those in the Rockies. This seems to leave only the alternative supposition that if the sediments came from the west they originated in distant mountain ranges which then existed off-shore where the deep Pacific Ocean now lies. Is this one of the instances Nevin and Kuenan say are “frequently” encountered where existence of former mountains must be “assumed” in order to provide a source of sediments which filled inland basins and synclines? H. G. Richards, in Record of the Rocks, p. 117, adopts such an assumption in demarking a source of sediments which filled the Cordilleran Geo-syncline. He says: “To the West [of the Cordilleran Syn-cline] . . . was the land of Cascadia, which extended westward into the present Pacific Ocean.”


Thomas R. Henry, writing in The White Continent about coastal mountains which were explored in Antarctica, says:

They are built up of layer after layer of sedimentary rocks. . . . The rocks, approximately 15,000 feet in thickness, are old ocean bottom; they are the compressed, petrified erosion of a continent, swept into the sea by floods, rivers and winds over millions of years.

He makes no attempt to designate the location of the highlands from which the sediments came, except to say that they came from erosion of a “continent.” He does not state whether he believes that the sediments came from the continental side or from the oceanic side of Antarctica.

Tagged on: , , , ,