PHYSIOGRAPHY OF THE BEAVERDELL MAP-ARIA. 
45 
valley of the West Fork, Wallace draw, and the lower parts 
of Trapper creek, Wilkinson, and Beaver creeks, were occupied 
by valley glaciers at one time. No cirques were found in the 
Beaverdell map-area, and it is inferred that the valley glaciers 
which moved down Trapper and Wilkinson creeks and the West 
Fork river were derived from higher ground to the north. The 
Beaver, however, rises upon comparatively low ground and its 
headwaters show no evidence of a nive-field of snow accumu- 
lation. A beautiful example of the U-shaped valley occurs 
at its lower end and this may have been formed by the glacier 
which occupied Wallace draw and left evidence of its presence 
in the moraine which lies at the point where Wallace draw enters 
the Beaver. 
The other topographic effects of glaciation in the valleys 
were the scouring out of rock basins like lower Arlington lake, 
the deposition of glacial material, and changes of drainage or 
stream piracy. Examples of glacial deposits are the moraines 
in Wallace draw, and in the draw south of lower Collier lake. 
Stream Piracy. 
Evidences of changes in the courses and volumes of streams 
are many. They seem to be largely due to the disorganizing 
effect of glacial deposits. The more usual occurrence of changes 
of stream volume is where at the junction of two streams, the 
smaller valley carries the larger stream ; for examples : the junct- 
ion of Trapper creek and the West Fork, and the junction of 
Beaver and Maloney creeks. In extreme cases one of the val- 
leys may be dry, having evidently been robbed of its entire flow; 
such is the case in Wallace draw and in the trough in which the 
lower Collier lake lies. Wallace draw and the lower portion 
of Beaver creek into which it opens, together form a valley 
trough as large as the corresponding part of the West Fork 
valley. But although they are all developed in the same type 
of rock Wallace draw i§ entirely dry and the lower end of the 
Beaver carries a comparatively small stream. Wallace draw, 
which is separated at its upper end from the floor of the West 
Fork valley by a divide only 40 feet high, was undoubtedly at 
one time the channel of the West Fork river. 
