THE EROSION AND GRAVEL ACCUMULATION. 265 



tween granite and andesite along which the section is laid runs for many 

 miles nearly due eastward across the divide, thus exposing an excellent 

 profile of the Neocene surface. The high volcanic ridges of mount Lola 

 immediately northward are projected on the section. The slopes of the 

 flows are eastward and westward from the central vents of the old vol- 

 cano of Lola. 



In L L a section is made on a smaller scale across the divide showing 

 the depression of the headwaters of the Neocene middle fork of the 

 American river, east of which rises the old divide at Granite Chief. The 

 river heads only a few miles northward and rises rapidly in this direction. 



The decided slope eastward from < Iranite ( Lief should be noted. That 

 this slope was also that of the Neocene divide is proved by the contact 

 lines of the andesitic masses with the underlying older rocks. This line 

 is projected on the section from the ridges immediately north of it. 



From the ragged country in the region of their sources the rivers 

 pursued their course down in broad valleys separate 1 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 com- 

 paratively 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 hack in the Cre- 

 taceous 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 con- 

 tinued in places with great activity and furnished some of the materia] 

 deposited in the lower parts of the streams. The coarse character of 

 much of the gravel and the often remarkable absence of tine sediments 

 in the beds point clearly to a somewhat rapid stream capable of carry- 

 ing off a great deal of silt, and the accumulations are probably due to 

 rapid overloading rather than to low grade of the rivers. The deep chan- 

 nels were filled and the gravels encroached on the adjoining slopes, where 

 they were deposited in broad benches. A maximum thickness of 500 feet 

 of deposits was attained on the South Yuba, and of from 50 to 200 feel in 

 the other parts of the lower rivers. En the lower and middle Siena some 



XL-Bum,. Geoi,. Soc. Am., Vol.. 4, 1802. 



