250 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
RESUME OF GLACIAL ACTION. 
The evidence goes to show that at a relatively remote period the Aspen 
district was covered by a great ice sheet, which moved west, away from the 
Sawatch range, over hill and valley. This glacier carved the surface into 
typical glaciated, rounded, and drumlinoid forms, and excavated the softer 
shales and sandstones more than the resistant granite. The débris of this 
ereat glacier is found on top of the highest mountains in the district: 
Subsequently this ice sheet shrank into separate glaciers, which followed the 
valleys of preexisting streams and in large measure carved them into their 
present forms. ‘These valley glaciers, by erosion along their sides, caused 
a steepening of the mountain slopes, and so brought about the removal 
of most of the previously accumulated drift. It thus happens that at 
present the sides of the mountain often appear bare and unglaciated, and 
the drift of the earlier glaciation is found only on the summit. At a still 
later stage in the glaciation the valley of Roaring Fork was occupied by a 
long, narrow glacial lake, which probably resulted from some temporary 
dam. During most of its existence the surface of this lake was 400 or 500 
feet above the present town of Aspen, and into it the dying glacier of 
Hunter Creek and its waters emptied. 
