42 GLACIAL AND SUEFACE GEOLOGY OF THE PACIFIC COAST. 



than 9,000 feet in elevation, and the ice in this portion of the valley must 

 have formerly been over a thousand feet in thickness. Into this basin came 

 down branches of the ice-flow from the Mount Conness Ran2;e on the north, 

 from the pass just north of Mount Dana, formerly known as McLane's Pass, 

 and from Mono Pass on the east, and from tlie mass of Mounts Maclure and 

 Lyell on the south, this latter being the most extensive of all. The lateral 

 and medial moraines of all these difterent flows are as plainly visible on the 

 surface as if the ice had but just melted away and left them. They are 

 particularly well marked where the glacier coming down from Mono Pass 

 united with that descending from McLane's Pass. 



At Soda Springs the valley of the Tuoknnne begins to narrow gradually, 

 and two miles below commences the remarkable gorge which continues for 

 twenty miles as a grand caiion, with extremely precipitous walls from a 

 thousand to fifteen hundred feet high, between which there is hardly any- 

 where much more than room enough for the river to run over its rocky bot- 

 tom. The walls of this cauon, however, although very steep, are nowhere 

 perpendicular as in the Yosemite ; neither are there any of those sudden 

 perpendicular drops of its floor, such as we have where the Merced falls over 

 the squarely-cut granite edges in plunges of 600 and 400 feet at the Nevada 

 and Vernal Falls. The whole of this canon of the Tuolumne has been occu- 

 pied by a glacier, as both the bottom and the sides show abundant evi- 

 dences of glacial striation and polish. Detrital accumidations, however, are 

 extremely rare in it. The facts observed prove very clearly that the form 

 of the canon has been in no respect due to ice-action. Projecting edges have 

 been rounded and smoothed. But the ice-mass followed the channel pre- 

 viously prepared for it : it did not do the work of excavation itself A brief 

 study of the position of the glacial markings is sufficient to establish the fact 

 that the canon could never have been excavated by the glacial mass which 

 once filled it. 



If it be asked lioiv this is shown, the answer can be very easily given. 

 If the canon had been excavated by the movement of the ice with which it 

 is admitted that it was once entirely filled, it would of necessity show that 

 this was the case by presenting exclusively, everywhere in its course, the 

 characteristic features due to ice-work. To use a homely illustration : if one 

 were to work out a trough in a plank with sand-paper alone, the resulting 

 cavity could not have angular recesses and projecting edges in it, for sand- 

 paper is a tool or material which cannot originate forms of that kind. So, in 



