GLACIAL GEOLOGY OF THE COHOES OLTADRANGLE 29 



In both cases, however, the "marginal slope, de'erminnig the front 

 of the terrace, is due to downcutting by currents shifted from a 

 broader to a narrower range of erosion consequent upon the sub- 

 sidence of the Lake Albany waters. 



The downcutting which attended the renewed subsidence of the 

 lake waters is definitely recorded in the steep slopes which bound 

 the erosion, terraces at their inner margins. As shown by the 

 contour lines of the sheet the slope, or bluff, of the west terrace 

 (facing the flattened area on which Mechanicville is built) is 60 

 feet in height and the bluiT of the east terrace (ex;;ending northwest- 

 erly from Reynolds) is 80 feet. The trend of the lines of these bluffs 

 is quite evidently such as would be caused by currents issuing from 

 the Anthony kill channel. And these currents must have been of 

 considerable power, seeing that their force was not spent in their 

 diagonal course across the lake waters. It is therefore inferred 

 that the Iroquois-Mohawk was still discharging into Lake Alban}- 

 at Mechanicville when the waters of the lakd had subsided to 

 the level of the eroded area at the foot of the bluff west of 

 Mechanicville. The eroded area just referred to has the physio- 

 graphic form of a terrace normal to the river at the present level 

 of its channel. The corresponding terrace on the east side of the 

 river is the level tract extending toward the river from the foot of 

 the bluff running northwesterly from Reynolds. These symmetrical 

 terraces may be designated the lower erosion terraces in contra- 

 distinction to the upper erosion terraces described above. 



EVIDENCE BEARING ON PO'STlGLACIAL HISTORY OF 

 HUDSON-CHAMPLAIN VALLEY 



These topographic features, due to erosion by the Iroquois- 

 A'lohaw^k 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 Cham- 

 plain valleys. The outlet of the interior glacial lakes (Algonquin- 

 Iroquois stage) continued through the Mohawk valley until such 

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

 slope of the Adirondacks. It "follows that the subsidence of Lake 

 Albany had proceeded to the extent that its waters had lowered 

 from the 360-foot level (that of the Hoosic delta) to the lOO-foot 

 level (that of the lower erosion terraces at Mechanicville) 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 



