322 ELIOT BLACKW ELDER 



in varying degree. In some cases the ice descended as far as the 

 foothills of the range, and a few of the largest ice-tongues expanded 

 out upon the adjacent plains. It is not my purpose to map or 

 describe in detail the glacial phenomena of each range, but rather 

 to discuss the general nature and course of events in the district as 

 a whole, using individual cases for illustrations and as evidence. 

 It is clear that the time which has elapsed since glaciers first 

 appeared in this region has been very long, and the time since they 

 vanished very short. On the one hand, there are deposits of till 

 that have been so deeply eroded that only remnants of them now 



Fig. 41. — The floodplain of Fish Creek in the Mount Leidy highlands. Benches 

 on the left are remnants of the older (Circle) floodplains, cut across inclined beds. 



remain, and the intervening canyons are hundreds of feet deep in 

 solid rock. On the other hand, some of the glacial features are as 

 fresh as if they had been made within historic times — the moraines 

 almost untouched by erosion, the bowlders essentially unweathered, 

 the lakes not yet filled, and the striated rock surfaces still con- 

 spicuous. In the older drift little if any trace of original glacial 

 topography remains, and most of the surface bowlders have dis- 

 appeared either through weathering or through burial by eolian dust. 

 An examination of only one or two valleys in the district might 

 prompt the conclusion that there were two and only two stages of 



