LA PLATA MOUNTAINS. 339 



glaciated on one side. Fresh exposures from beneath the soil reveal gla- 

 ciated rock up to near the top of the secondary ridges. 



Moraines. — Ou tlie stecpcr lateral slopes of the valleys there .is no 

 moraine stuff. On gentler slopes there is a thin sheet or scattering of 

 erratics. No distinct ridges or terraces were observed, except one on a 

 shelf of the mountainside situated in the valley of the East Mancos River 

 about 3 miles from its head, and 250 feet above the valley; also two at 

 Helmet Peak. This peak is the highest peak of a high ridge which sepa- 

 rates the East Mancos and West Mancos valleys. To the west (in lee of) 

 this peak, and perhaps 50 feet below the summit, are two moraines, one 

 lateral to each valley. Most of the material is rather fine and well gla- 

 ciated. The upper surface of the moraines conforms to the slopes of the 

 mountain, here quite gentle. These terraces are shown to be moraines, 

 not only by the glaciation of the stones, but also by the fact that the 

 local rock is igneous (hornblendic trachyte of Hayden) while most of the 

 glaciated stones are of quartzite and other erratic material. The upper 

 part of the peak is so weathered and shattered that I can not be sure 

 whether it was ever glaciated or not; hence it is uncertain whether these 

 moraines were pushed out laterally at the surface of the ice or were formed 

 subglacially as tail to the peak as crag, at a time when the ice in the two 

 valleys rose above the ridge that divides them. This is about 600 feet 

 above the valley of the East Mancos. 



In the upper 3 miles of the valley of East Mancos River there are a 

 number of small retreatal or terminal moraines in the bottom of the valley, 

 which here is U-shaped, but becomes V-shaped nearer the plains. After 

 entering the plains the stream flows in a valley of erosion in sedimentary 

 beds. This valley grows wider and wider up to near a mile in breadth 

 at Mancos. 



Lateral and terminal moraines would naturally form where the stream 

 emerges from the mountains, but the slope here is very steep, and most of 

 the moraine stuff appears to have become a part of the glacial gravel or 

 has been left much scattered The upper valley of La Plata River is 

 considerably broader than that of the East Mancos. It contained a much 

 larger glacier, which has left moraines arranged about like those of the 

 valley already described. 



Glacial gravels. — Large ovcrwasli or frontal aprons of water-rounded gravel, 



