GLACIATION OF WRANGELL MOUNTAINS 37 



Since the ice-borne materials are either deposited at the melting 

 edge of a glacier, or beneath the body of the melting portion of the 

 ice, there is always a great deal of running water present. Often 

 streams of large volume flow out from beneath the glacier, but the 

 volume of flow varies seasonably and daily as the temperature rises 

 and falls. At times of rapid melting the streams carry large volumes 

 of water, and are able to handle a great amount of the debris brought 

 down by the ice. The material may be carried for long distances, 

 or much of it may be dropped within a short distance from the glacier. 

 The daily fluctuations in volume of the streams is an important 

 factor in both the transportation and deposition of the debris. 



Materials which have been deposited by streams differ notably 

 in structure from those deposited directly by the ice. The water 

 tends to assort the materials, and while the stratification may be very 

 imperfect, the structure is readily distinguishable from that of glacial 

 till. 



Both the moraines and the stream-laid gravels form important 

 topographic features in the valleys below the glaciers. The moraines 

 are usually most prominent near the ice-edge, as they are readily cut 

 away and destroyed by the streams. The outwash gravels are often 

 of great extent, and the broad gravel bars with their anastamosing 

 streams cover the valley floors of almost all the glacier-fed drainage 

 lines. 



EVIDENCES OF EARLIER AND MORE EXTENSIVE GLACIERS 



We have seen that glaciers have an important influence upon the 

 valleys which they occupy, both in determining the shape of the 

 valleys, and in causing the deposition of moraines and gravels.- 

 These evidences are definite, and would remain even if the ice should 

 melt and disappear. In the region under discussion there is abundant 

 evidence of this sort, which shows that at no distant geological period 

 the glaciers were of much greater size and extent than they are now. 

 The valleys have been broadened and deepened, and show a marked 

 U shape in cross-section far below the limits of the present ice. 

 Furthermore, the rock surfaces are often striated, and there are unmis- 

 takable deposits of glacial till at many points from which the ice has 

 long ago disappeared. In the Nabesna Valley, for example, the ice 

 probably extended 40 or 50 miles to the northeast from the edge 



