PLEISTOCENE GEOLOGY OF THE SAWATCH RANGE 699 



age, with possibilities of a third epoch much earher than the two 

 whose resuks are weh marked. 



EXTENT OF ICE 



The last glacial epoch. — The ice of the last glacial epoch covers by 

 far the larger part of the surface of the mountains proper. The ice 

 came down all the larger valleys tributary to the main valley, and in 

 most cases only the narrow crests of the ridges between the side 

 valleys projected above the glaciers. In several instances the glaciers 

 of the side valleys descended to the present position of the river in 

 the main valley, and in a few cases they crossed the main valley to 

 the base of the opposing range. Of the entire area west of the great 

 north and south valley about three-fourths was covered by the ice 

 of the last glacial epoch, and a large part of the unglaciated one- 

 fourth lies beyond the foot of the mountains proper. 



Ten distinct systems of glaciers of some importance were studied 

 in this area, besides four chff glaciers of small size. Of the ten 

 more important systems three extended barely to the foot of the 

 mountains. Seven got well out into the wide valley, east of the 

 mountains, and live of them extended to the present position of the 

 river, and the same number extended below the 9,000-foot contour. 



The glaciers varied in length from one to twenty miles, and in 

 area from one-third of a square mile to about eighty square miles. 

 The Lake Creek system was longest, and also greatest in area. Of 

 the very small glaciers, one started at an elevation of 11,000 feet, 

 but an elevation of about 12,000 feet seems to have been necessary to 

 start glaciation in most places. The glaciers all originated in similar 

 situations. Their sources were in cirques which lay back toward 

 the higher parts of the mountains. The ice from these cirques, 

 moving downward, merged in each of the principal mountain valleys, 

 and developed a tongue of ice which advanced far down the valleys. 

 About sixty separate cirques contributed to the ten large glaciers of 

 the region. 



In all the larger glaciers the ice had a thickness of over i ,000 feet, 

 while in the valleys of Homestake and Roche Moutonnee Creeks 

 the thickness exceeded 2,000 feet. 



The drift. — The character of the glacial deposits varies greatly in 



