3 oo LEWIS G. WESTGATE 



other tributaries which were occupied by valley glaciers show the 

 same general features. See in this connection Fig. 10. 



The main valley above Twin Lakes shows the same broad U -cross- 

 section, but in the matter of broad detail there are serious interrup- 

 tions to the regular profile, in rocky barriers across the stream-floor, 

 and in some cases in buttresses standing out prominently from the 

 valley side. The longitudinal profile of the valley is not even. For 

 a distance the stream may be swinging from side to side in an alluvial 

 valley floor, and then may for a fraction of a mile be flowing with 

 more rapid fall in a rock-gorge between the low, rolling, glaciated 

 hills of one of the valley-sills. Four such sills occur in the main 

 valley within a distance of six miles above the mouth of the rock- 

 gorge. The largest is the one at the mouth of the rock-gorge. It 

 consists of a series of rounded rock-bosses rising to the higher walls 

 of the valley on either side, but is most conspicuous on the north, 

 where it forms a long rocky barrier, reaching nearly to the upper 

 limit of glaciation. The difference in level of the stream between 

 where it enters this rocky portion of its valley above, and where it 

 leaves it below, is 600 feet; and it is here flowing for a mile in a 

 narrow postglacial gorge. Other sills occur at several points up- 

 stream, but are not as conspicuous as the one at the mouth of the 

 rock-valley. 



Rock-bosses standing out from the valley wall also occur. The 

 two on either side of Monitor Gulch are the best — Monitor Rock 1 

 on the west side, and another nearly as high on the east. Monitor 

 Rock rises nearly 1,500 feet above the valley floor, nearly to the 

 upper limit of valley glaciation, and stands out conspicuously into 

 the valley. 



An even U -trough is the ideal toward which the glacier tends to 

 shape its bottom; but rock-sills and buttresses show that in the 

 process of valley-cutting the glacial bottom may be fashioned quite 

 irregularly, on account of differences either in rock-resistance or in 

 glacial erosion at different points. Monitor Rock may be due to 

 difference in rock hardness, as it is composed in part of a finer 

 grained, denser, and probably more resistant fades of the porphyritic 

 granite which is the common rock of the valley. But it is not pos- 



1 Described by Hayden, Annual Report for 1873, p. 54. 



