GLACIAL GEOLOGY OF THE SCHENECTADY QUADRANGLE II 



the lake; beyond this the floor of the channel is continued to the 

 base of the slope. Its surface shows many minor elevations and de- 

 pressions. The elevations are in part due to the vertical position of 

 the strata which in places appear as ridges where beds of sandstone 

 alternate with the softer shales. Moreover, on both sides of the 

 lake the rocks are horizontal in position. Thus erosion appears to 

 have taken place more destructively across the faces of the rocks 

 than on their edges. Glacier ice, with its inclosed fragments of 

 hard rock, would more probably produce this condition than would 

 running water. In this connection the somewhat remarkable depth 

 of the lake at its southern end merits consideration. A sounding 

 taken by the writer at a point about 400 feet from the southern 

 end of the lake showed a depth of 109 feet. Another sounding 

 taken at a point about half a mile from the southern end showed 

 a depth of 55 feet. This narrow and deep depression somewhat 

 abruptly terminating at the head of the lake, suggests the gouging 

 action of glacier ice rather than river erosion. 



The general direction of the axis of the channel, coinciding well 

 with the direction of the glacial striae of the general region, is in 

 harmony with the view here expressed. The deductions made from 

 these data are as follows : 



It is highly probable that a preglacial stream heading to the 

 north followed the course of the present Ballston channel, finding 

 its bed in the line of the vertical outcrops of rock, and joined the 

 Mohawk near Schenectady. The valley made by this stream was in 

 glacial times scoured out and enlarged by ice erosion. At a later time 

 as will be explained farther on, after the melting of the ice, power- 

 ful currents, diverted from the flooded Mohawk, swept northward 

 through the open channel, producing some further erosive effects 

 and finally, with the passing of the flooded epoch, leaving the chan- 

 nel in its present features. 



THE MOHAWK GORGE 



As already stated the Mohawk river beginning near Aqueduct 

 occupies a channel which has been formed in postglacial times. 

 The evidences of this may be briefly stated as follows : An abrupt 

 change in the features of the valley begins just west of Aqueduct. 

 From a broad valley, with (on the north side) a gentle slope and 

 wide flood plain, there is a transition to a narrow, deep valley or 

 gorge. The cliffs on either side, below Aqueduct, are nearly vertical 

 and about 140 feet in height. There are no high, gently sloping hills 



