THE PLEISTOCENE OR GLACIAL PERIOD. 



471 



glaciation was extensive, derangement of the drainage was common, 

 and deposits of glacio-lacustrine clay, himdreds of feet deep, are known 

 at some points. Where such deposits were made in narrow valleys 

 now drained, they have been partly removed, and their remnants con- 

 stitute terraces. 



Topographic unconformity. 1 — Glaciation in the west was also respon- 

 sible for a phase of topography worthy of special mention. It is illus- 

 trated by Fig. 553. A great glacier passed down through the valley, oblit- 

 erating the erosion topography of its lower slopes, partly by wearing 

 away the ends of the ridges between the tributary valleys, and partly 

 by filling the lower ends of those valleys, up to the limit of the ice. 



Fig. 553. — Topographic unconformity developed by glaciation, and by a glacial 

 lake. Lower end of Lake Chelan, Wash. (Atwood.) 



The result was that the well-developed drainage lines on the upper 

 slopes were effaced below, and post-glacial erosion has since developed 

 new channels in this part, continuous with the older ones above, thus 

 giving rise to a topographic unconformity. In the case shown in 

 Fig. 553 the lake (Chelan) stood at the levels of the terraces after 

 the ice disappeared, and its shore deposits helped to destroy the lower 

 ends of the preglacial drainage lines. Fig. 551 also shows topographic 

 unconformity. 



All evidences point to the conclusion that the glaciation, or at 

 least the latest glaciation, of the western mountains was of very recent 

 date. From a general study of the data at hand, it would appear 



1 Jour, of Geol., Vol. XII, p. 707. 



