THE TWIN LAKES GLACIATED AREA 



301 



sible to make any general statement as to the greater hardness of 

 the rock barriers, because much the same kind of rock is found all 

 along the valley. In the present case these irregularities occur well 

 below the limit of hanging valleys, and so in that part of the valley 

 excavated by the glacier; they are not due to subaerial erosion, 

 though the prominence of projecting buttresses may have been 

 so increased. 



The separation of the different elements of the valley side indi- 

 cated in Fig. 4 is difficult, on account of weathering and of the indefi- 

 niteness of the upper limit of glaciation. The upper preglacial 

 slope is easily recognized in the rounded summits of the divides 



Fig. 5. — Monitor Rock, Lake Creek Gorge, looking down the gorge. 



between the tributaries to Lake Creek. It is a graded slope of 

 about io°, is usually grassed over, and is shown where the divides 

 between the tributaries have not been sharpened by glacial erosion. 

 But it is not easy to distinguish the cliff of glacial abrasion from the 

 oversteepened cliff above the former glacial bed. The glaciated 

 valley surfaces have been largely destroyed by subsequent weathering, 

 and where the height of the glacier was changing, it may easily have 

 been that the passage from a well-glaciated wall below to a non- 

 glaciated wall above was originally gradual and indefinite. In the 

 lateral valleys weathering has practically destroyed all the glacially 

 smoothed surface of the valley walls, and the upper limit of glacial 

 erosion can be made out only approximately. In the main valley 

 the upper limit can be more definitely fixed, and while even here 

 there is doubt about its exact location, a lower portion of the valley 

 wall can be recognized, showing under favoring conditions well- 

 developed glaciated surfaces, while an upper, steep, rocky portion 



