Page:Popular Science Monthly Volume 52.djvu/633

Rh of 1872. West of Mono Lake the total displacement can not be less than six thousand feet, while farther south in Owen's Valley it must have reached eight thousand to ten thousand feet, if not more.

Following the first faulting in the Cretaceous outlining the region occupied by the Sierra Nevada Mountains, the erosive agencies began to work actively, for the area was undoubtedly high and rugged. In the course of time the mountains were worn down in places to an approximately base-leveled condition, and erosion almost ceased. As a result of the renewed elevation in the latter part of the



Tertiary, rapid erosion was again inaugurated and the present deep canons were excavated. While these canons on both slopes are exceedingly rugged, those on the east are very short, owing to the fact that the crest of the range almost overlooks the fault scarp. Those crossing this scarp have cut down nearly or quite to the level of the upper part of the débris fan which has grown up at their mouths. They are three thousand to five thousand feet deep, terminating upward in cirquelike amphitheaters of precipitous rock. Through the more rugged portion of the range the mountain peaks have in many instances flattened, plateaulike summits which present a