420 j. m. stoller topographic features of hudson valley 



Deductioxs 



These topographic features, due to erosion by the Iroquois-Mohawk 

 currents, afford a datum for determining the time of subsidence of Lake 

 Albany with reference to the location of the front of the ice-sheet in its 

 retreat from the Hudson and Champlain valleys. The outlet of the in- 

 terior glacial lakes continued through the Mohawk Talley until such 

 time as the receding ice-front opened an outlet along the northward slope 

 of the Adirondacks. It follows from the data above given that the subsi- 

 dence of Lake Albany had proceeded to the extent that its waters had 

 lowered from the 360-foot level (that of the Schenectady delta) to the 

 100-foot level (that of the lower erosion terraces at Mechanicsville) 

 within the period of time the ice-front was in process of retreat to the 

 northern end of the Champlain region : for during the continuance of the 

 Iroquois-Mohawk outlet its currents lowered their bed in the Lake Albany 

 deposits, pari passu with the subsidence of the lake waters, from the sur- 

 face of the deposits to the level of 100 feet. 



There is no evidence that the waters of the Hudson Yalley, after their 

 subsidence to the level indicated by the lower-eroded areas at Mechanics- 

 ville, rose again to a higher level. The present major topographic fea- 

 tures of the valley — the terraces and their slopes — have certainly not been 

 modified by overflowing waters since their origin. If, therefore, a water 

 connection existed between the Champlain arm of the sea and the ocean 

 at Xew York, this strait had a breadth at Mechanicsville not greater than 

 approximately the space between the 100-foot contour lines on the oppo- 

 site sides of the valley — that is, a breadth not greater than that of the 

 present valley bottom. 



These deductions, drawn from the facts of topography, are opposed to 

 the conception of a body of marine waters filling the Hudson Yalley to a 

 height indicated by the river deltas and continuous with marine waters 

 in the Champlain depression. 



If we suppose that the Hudson Yalley waters in which the sands and 

 clays were de$)osited were estuarine, opening into the sea at Xew York, 

 then that portion of the estuary included in the region of Mechanicsville 

 had become changed to a fresh-water river while yet the Saint Lawrence 

 basin was filled with ice. We are thus led to conclude that at no time 

 was there a continuous body of marine waters connecting the Saint Law- 

 rence arm of the sea with the ocean at Xew York. 



