We shall now turn to some large scale cases of faults in which great masses of earth have been pushed one over another—so-called “thrust faults.” In the southern Appalachian Range, and especially well exhibited in the vicinity of Rome, Georgia, one portion of the mountain mass has literally been shoved over another, at a low angle over a fault surface many miles long, for fully seven miles westward. Both the tremendous weight of rock material actually translated and the number of sudden movements required in the operation stagger the imagination. It is safe to say that during the long time of this great operation violent earthquakes were not uncommon. In the Rocky Mountains of the northern United States and southern Canada there is the greatest known thrust fault on the continent. It is hundreds of miles long, and the actual displacement is commonly at least several miles. In the Glacier National Park of Montana it has been established that the front range portion of the Rockies has actually been pushed at least seven miles, and possibly as much as twenty miles, eastward over a fault surface, and out upon the Great Plains. In some cases rocks of the Prepaleozoic Age have there been pushed upon rocks of the late Mesozoic Age, thus locally upsetting the geologic column.
Fig. 11.—East-west profile and vertical structure sections fifty-two miles long in the Mohawk Valley region of New York, showing numerous tilted fault blocks which notably influence the topography. Vertical scale exaggerated. The rocks are Prepaleozoic and early Paleozoic in age. (Modified by the author after Darton, New York State Museum.)
The Wasatch Range of Utah, in addition to the great normal fault along its western base, contains a remarkable system of thrust faults. In the region now occupied by the Wasatch Mountains a number of parallel (thrust) faults were developed close together and the broken pieces of the earth’s crust between them were pushed up, the rocks on one side of each crack riding up over those on the other side until a great mountain range was formed where once lay a plain. In the Ogden Canyon one great earth block of Prepaleozoic (Algonkian) Age has been shoved thousands of feet over late Paleozoic (Carboniferous) rock, which latter has in turn been thrust over early Paleozoic (Cambrian) rock. This thrust faulting was accomplished before the development of the geologically recent normal fault along the western base of the range.
Fig. 12.—Vertical (structure) section through a part of the earth’s crust several miles long in Ogden Canyon, Utah, showing the system of great thrust faults. Prepaleozoic (Algonkian) rocks have been pushed far over upon late Paleozoic (Carboniferous) strata, which latter have in turn been shoved over early Paleozoic (Cambrian) strata, etc. (After U. S. Geological Survey.)