TOPOGRAPHY OF THE PACIFIC COAST.
51
follows: their beds are narrower, rims steeper, and accumulations of bed rock gravel incomparably smaller." In these large accumulations of older gravels Prof. Whitney saw evidence of larger streams and heavier precipitation during the gravel period than now belongs to that region,[1] but, as pointed out by Mr. Gilbert,[2] deposition in stream channels is indicative of diminished instead of increased rainfall.
Professor Le Conte regarded the gravels as "deposits made by the turbulent action of very swift, shifting, overloaded currents" supplied with both water and debris "by the rapid melting of extensive fields of ice and snow" which were then supposed to occupy the higher portion of the range.[3]
A very important contribution to the literature of the auriferous gravels has been made lately by Mr. W. Lindgren, whose views are expressed in the following quotation:[4]
"From the rugged country in the region of their sources the rivers pursued their course down in broad valleys separated by ridges which even in the lowest foot-hills sometimes reached an elevation of a thousand feet above the channels. The outlines of the ridges were usually comparatively gentle and flowing; still, slopes of ten degrees from the channel to the summit were common and slopes as high as fifteen degrees occurred in the eastern part of the Sierra. The character of a region of old and continued erosion, commencing probably far back in the Cretaceous period, is everywhere plainly evident. In the center of the deep depressions is quite frequently found a deeper cut or "gutter," indicating a short period of more active erosive power just before the beginning of the gravel period. At this time, probably about the beginning of the Miocene period, the streams became charged with more detritus than they could carry and began to deposit their load along their lower courses, especially at places favorably situated, as, for instance, along the longitudinal valley of the South Yuba. Toward the close of the Neocene, gravels had accumulated all along the rivers up to a (present) elevation of about 5,000 or 6,000 feet; above this it is plain that erosion still continued in places with great activity and furnished some of the material deposited in the lower parts of the streams. The coarse character of much of the gravel and
- ↑ Climatic Changes in later Geological Times, p. 1.See also Auriferous Gravels, p. 335.
- ↑ Science, Vol. I., p. 194, March 23, 1883.
- ↑ Am. Jour. Sci., Vol. XIX., 1880, p. 184.
- ↑ Bul. Geol. Soc. of Am., Vol. 4, pp. 265-6.