308 LEWIS G. WESTGATE 



glaciers and became U -valleys, the shorter being hanging V-valleys. 

 During the succeeding interglacial period the tributary streams cut 

 back the mouths of their hanging valleys for a greater or less distance, 

 forming V-valleys or ravines at the lower end of the U -valleys, and 

 deepening the V at the lower end when the valley had not been 

 previously glaciated. The glacier of the second period smoothed 

 the walls of these interglacial stream-cut re-entrants where they were 

 not too sharp, but in practically all cases dumped more or less 

 morainic material into them. Postglacial time has enabled the streams 

 for the most part to cut through this drift, and below the mouth of 

 the hanging valley they follow a steeper course, chiefly in morainic 

 debris, but occasionally cutting into the solid rock. This interpre- 

 tation of the phenomena at the mouths of the hanging valleys implies 

 that the larger features of the forms of the glacial valleys were assumed 

 in earlier glacial time — an inference in accord with the evidence of 

 the moraines. 



POSTGLACIAL CHANGES 



Since the final disappearance of the ice, many changes of minor 

 importance have taken place, in the way of both erosion and deposi- 

 tion. Streams flowing across drift and across rock barriers in the 

 valley of Lake Creek have cut their channels. Behind the recessional 

 moraines of Lake Creek considerable deposition has taken place, 

 forming the extensive meadow at the head of Upper Twin Lake, 

 and smaller patches of bottom-land further up-stream. The most 

 conspicuous changes, however, are the rock-weathering and land- 

 slides along the main valley of Lake Creek, but especially along and 

 at the head of its tributary valleys. The almost entire destruction 

 of glacially smoothed surfaces in the tributary valleys has been 

 mentioned. Here, at altitudes near and above the timber line, 

 there has been a steady rain of rock fragments from the valley sides, 

 and talus cones are everywhere present, mantling the lower parts 

 of the gulch walls, and in many cases merging to make a nearly 

 continuous talus slope. In addition to this gradual accumulation 

 of fallen blocks to make a talus slope, great masses of broken rock 

 have fallen from the sides and shot out to or beyond the axis of the 

 valley. In one case, in the upper end of Willis Gulch, for a distance 

 of half a mile along the valley side, and for a height of 1,000 to 1,500 



