POST-TERTIARY ELEVATION OF SIERRA NEVADA 541 



A view of this cross- section of the old Neocene Tuolumne channel is given on 

 plate 58. 



Since between this locality and mount Dana the bed rock series is all granite, it 

 appears probable that in Tertiary time, as now, the Tuolumne river headed near 

 mount Dana. Where Rancheria creek runs through Deep canyon, it has but a 

 slight grade, which is probably nearly the grade of the Neocene Tuolumne, which 

 formerly passed through it. The water for considerable stretches is quite still 

 late in summer, when the flow is small. 



Although none of the gravels or the lavas of the Neocene Tuolumne basin are to 

 be found between Rancheria mountain and a point north of Poopenaut valley, 

 nevertheless the approximate course of the channel is not a matter of doubt. 



The configuration of the country is such that the river must, as before noted, 

 have either gone down Rancheria creek or over the site of Tiltill valley, thence 

 westward over the site of the Hetch Hetchy. The lava patches on the ridge-north 

 of Poopenaut valley are presumed to rest on a portion of the slope of the Neocene 

 Tuolumne basin, and the same is true of the lava area 3 miles west of Poopenaut 

 valley, and the gentle slopes of the ridge in this vicinity are doubtless a portion of 

 the same basin. The next point where the lavas are preserved is about 4 miles 

 westerly from Hog ranch. From there still farther westward there are other lava 

 patches, some of them capping river gravels. 



On Rancheria mountain, resting on andesite-tuff, and apparently capped by the 

 compact lava (latite) adjoining, is some gravel containing pebbles of augite- 

 andesite, pegmatite, quartz. This evidently represents a stream of the volcanic 

 period, and later in age than the gravels above described. 



The most western point where the gravels of the Neocene Tuolumne have been 

 preserved is east of the head of Big Humbug creek, in the Sonora quadrangle, and 

 the most eastern Piute canyon. If now we calculate the average grade of the 

 Tertiary stream between these two points, and the average grade of the present 

 river between the same points, we can compare the grades of the two streams. 

 The altitude of the Neocene Tuolumne gravels at Big Humbug creek is about 2,800 

 feet, and at Piute canyon 7,500 feet, giving a difference of 4,700 feet. The altitude 

 of the present Tuolumne north of Big Humbug creek is 1,500 feet, and at Pate val- 

 ley, at the mouth of Piute creek, 4,550, giving a difference of 3,050 feet. The hori- 

 zontal distance between the two points is about 33 miles. 



Assuming that both the Neocene and the present streams took a direct course, 

 we have a grade of 142 feet to the mile for the Neocene channel and a grade of 92 

 feet to the mile for the present channel. While the Neocene river occupied a 

 rugged canyon, nevertheless this canyon was much less deep and rugged than that 

 of the present Tuolumne, which implies, other things being equal, a higher grade 

 for the present than for the Neocene channel, while, as we have seen, the reverse 

 is the case. The broad channels and large sand and gravel deposits of the Neo- 

 cene streams of the Sierra farther north can scarcely be explained on any other 

 hypothesis than of comparatively gentle grades indicating an old age for the 

 streams, and this must have been likewise true of the Neocene Tuolumne, although 

 in less degree. 



Assuming that the Neocene Tuolumne had originally a grade at least as low as 

 that of the modern stream, which is evidently yet a young stream, it is clear that 

 the present grade of the Neocene channel must have been brought about by a dif- 

 ferential uplift on the east, resulting in a tilting of the range westward. 



