GLACIAL AND POST-GLACIAL HISTORY 633 



In the process of down- cutting the river terraces which occur in 

 the upper Hudson and on the tributary streams in both upper and 

 lower Hudson, were made. Some river terraces had also been made 

 in the tributary valleys before the close of the history of the Hudson 

 water body. 



Before the close of Higher Glacial Lake Champlain history, it is 

 believed, the depression which has drowned the lower Hudson and 

 its tributaries had begun. The basis of this belief is the amount of 

 filling of the southern Hudson since the submergence. While this 

 is a matter subject to revision on more accurate knowledge, calcula- 

 tions made indicate that the contributions of the Hudson and its 

 tributaries since the submergence would be inadequate to furnish 

 the material for this filling, and therefore that some of it was supplied 

 by the cutting of the trench in the old lake-floor or old sea-floor in 

 the northern Hudson Vallev. before the Fort Edward outlet was 

 abandoned. 



POST-HUDSON-CHAMPLAIN CHANGES OF DRAINAGE IN THE HUDSON 



VALLEY. 



By the close of Higher Glacial Lake Champlain history nearly all 

 the cutting by the Hudson south of Fort Edward had been accom- 

 phshed. This is shown by the fact that the Fort Edward outlet 

 floor is within less than 20 feet of the present Hudson level. 



Aside from the trenching of the consequent courses of the streams 

 below the floor of the Hudson water body, the pushing back of the 

 steep gradients from the neighborhood of the Hudson River bluffs, 

 and the development of subsequent tributaries on the consequent 

 streams, a part of which at least was accomphshed in Higher Glacial 

 Lake Champlain time, there have been few changes in the valleys of 

 the small streams since the disappearance of the Hudson water body. 

 Depression of the land, part of which probably took place before the 

 close of Higher Glacial Lake Champlain time, has drowned the lower 

 courses of many tributaries from Troy south, and gravel has been 

 carried out from the higher land into the trenches in the clay, making, 

 in some cases, a gravel-floor, and in others, by further erosion, a gravel- 

 floor and gravel-capped clay terraces, as on the Oniskethau. In 

 a few places it seems likely that readjustments in drainage have taken 



