CORRASION BY GRAVITY STREAMS. 303 



however, failed before the work of recession got beyond 

 the initial stages along the channels of these confluents. At 

 the summit of the Nevada Falls for the Merced, and just 

 below Tenaya Lake for the Tenaya valley, we see the 

 junction of the two glacial types, namely, the more mature 

 upstream phase and the upper limit of the vigorous youthful 

 recession. 



Had the time factor been increased, the strong quarrying 

 phase just described would have continued until the upper 

 Tenaya and Merced channels had become profound canons 

 of the Yosemite type. For example, with increased time 

 well shaped steps connecting the upper and lower valleys 

 would have been produced, the latter having flat floors 

 almost to the feet of the "steps." 



Below Inspiration Point the general cross-section of the 

 valley increases, and the channel slope is very great. The 

 glacier became ineffective as a corrader below El Portal, 

 and merely dumped moraine masses in its neighbourhood. 



There may be an apparent difficulty in associating this 

 decadence of glacial corrasion with the descent of a steep 

 channel slope. In these pages steep slopes have been sup- 

 posed to imply great increase of velocity. Nevertheless 

 in a locality situated far below the snow line and to which 

 the ice has been permitted to come only by reason of its 

 great volume, then any long and steep channel declivity 

 occurring below this level must result in glacial dissipation, 

 because of the necessarily rapid increase of temperature. 



Subsequently came a much weaker stage of glaciation, 

 but this seems to have had little effect on the Yosemite 

 Valley itself beyond dumping moraines in the old channel 

 which had been practically free from debris. These then 

 appear to be some of the main points in the history of 

 Yosemite as one sees it to-day. Among others, brief 



