420 STEPHEN R. CAPPS: 
in stream-developed valleys, and the truncation of the lateral spurs, 
giving straight-walled rock troughs. Where the valley makes a 
bend in its course the ice erosion has developed great sweeping 
curves so that the glaciers seldom bend at sharp angles. ‘These 
topographic features, with hanging tributary valleys and a large 
number of other characteristic and readily recognizable phenomena 
of glacial erosion, are of service in determining the thickness and 
former extent of the glaciers in those places where deposits of - 
glacial débris are not to be found. 
EVIDENCE OF EARLIER AND MORE EXTENSIVE GLACIERS 
The outer limits to which the glaciers of the Alaska Range 
reached at the time of their maximum extent have for the most 
part not yet been determined, and much careful detailed work will 
be required before this limit can be accurately marked. There 
are certain facts available, however, which show in a general way the 
area covered by the ice during its greatest development. On the 
south side of the range no limit of glaciation is shown on the accom- 
panying map (Plate I), for all of the area here shown except the 
higher peaks and ridges of the more important mountain ranges 
was covered by glacial ice. The entire Copper River basin was 
occupied by a great glacier, as has been recognized from abundant 
deposits of glacial till, which are found throughout the basin at 
points as far as possible removed from the mountains in which the 
ice must have originated. The Copper River ice sheet was of 
great. thickness, for it escaped from the basin in a number. of 
directions. It certainly pushed north across Delta Pass, and 
probably also to the northeast through Mentasta Pass. 
To the west it extended into the head of Susitna basin, as is 
shown by the widely distributed deposits of glacial till in that 
region. Matunaska Valley heads in a broad divide at an elevation 
of 3,000 feet, and probably furnished an outlet for the glacial ice 
to the southwest. The present drainage outlet of the basin, 
through the Copper River Valley, was also an outlet for a part of 
the glacial ice, although it is probable that this outlet was not 
the course followed by the pre-glacial streams and that it was not 
established till Glacial times. Near the mouth of Chitina River 
