Page:The Journal of geology (IA journalofgeology21894univers).pdf/61

This page has been proofread, but needs to be validated.
TOPOGRAPHY OF THE PACIFIC COAST.
47

surface of the Sierra Nevada has been deformed during this uplift, and that the most noticeable deformation has been caused by a subsidence of the portion adjoining the Great valley relatively to the middle part of the range."

Strong evidence of the deformation is furnished by the distribution of the Ione formation. As already shown, this formation was deposited about sea level. On Little Cow creek it now occurs at an altitude of 3,400 feet, and on Bear creek about 4,000 feet above the sea, indicating conclusively that since the baselevel period the Lassen Peak region has been elevated at least 4,000 feet. There are indications that the elevation was still greater to the southward about the northern end of the Sierra Nevada, for between Mountain Meadows and Diamond Peak opposite Susanville the auriferous gravels supposed to belong to the estuarine Ione formation rise from 5,000 to 7,000 feet. These high gravels upon the northeastern block of the Sierra Nevada have been displaced in a remarkable manner by the upheaval of the range. The area occupied by them is about 10 × 16 miles in extent. Although the gravels cover the larger part of this area and are connected throughout, they do not appear over the whole of it. There were a few small islands of older rocks during at least the later portion of the gravel period, and at some other places within the area the gravels have either been washed away or covered up by later volcanic flows.

During the later part of the gravel period in that region, after the effusion of the andesitic lavas, more or less well defined beaches were formed around a series of volcanic islands upon what is now the very crest of the range from Fredonia Pass northeast of Mountain Meadows to Diamond Mountain. When developed, these beaches must have been at the same level in a body of standing water, but now they gradually rise to the southward from about 5,000 feet near the northern end of Mountain Meadows to 7,000 feet opposite Diamond Peak, and it is evident not only that the northern end of the range has been elevated but that the amount of elevation increased to the southward. The general inclination of this body of gravels toward Lassen

This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.