GLACIAL AND POST-GLACIAL HISTORY 465 



In the southern Champlain Valley the tributaries bear evidence of 

 recent drowning of their lower courses. North-heading streams show 

 a recent revival, while south-heading streams show arrested develop- 

 ment. 



In the eastern passage to the Hudson a valley has been mentioned 

 which will be described in two divisions as the Fort Edward Valley 



Fig. 19. — Showing the relation of the joint structure in the Potsdam sandstone to 

 one of the rectangular turns in the Saranac River. 



[Photograph by W. S. McGee.] 



and the Whitehall-Putnam Station Valley. The tributaries flowing 

 into these valleys show characteristics similar to those of the Hudson 

 tributaries. They descend to the main valley, over steep slopes, 

 which have been pushed farther back on the larger streams and 

 reduced to gentler slopes than on the smaller streams, except where 

 the rock has been encountered. This character is to be contrasted 

 with the comparatively gentle gradients of the streams flowing into 

 Lake Champlain farther north, where the streams lack the sudden 

 descent at their mouths that is characteristic of those flowing into 

 southern Champlain or into the Fort Edward Valley, except where 



