﻿GEOLOGY OF THOUSAND ISLANDS REGION 149 



tude, which very likely was partly controlled by early glaciation, 

 yet a significant number are transverse. Some basins in the vicinity 

 of Alexandria Bay [pi. 47] and others south of Clayton [pi. 46] 

 do not conform to the prevailing direction, and the basins of the 

 Redwood lakes are so irregular in form as to rule out ice erosion 

 as the dominant agent. It seems certain that these basins, like the 

 scarp borders' of the plateaus, are due to atmospheric agencies 

 with only small and indeterminate glacial effects ; or that they cer- 

 tainly antedate the latest ice invasion. One would naturally sup- 

 pose that the scraping ice sheet would have rubbed the transverse 

 valleys full of drift. In some valleys and against some scarps the 

 amount of drift is sufficient to be noticeable, but it only masks the 

 foot of the cliffs. In many relatively deep depressions the drift 

 is scarcely perceptible, though some may be buried under the lake 

 silts which occupy the valley bottoms. 



Besides the lack of drift filling is to be noted the absence of pre- 

 glacial talus accumulations. In places the Potsdam is so freely 

 jointed that the cliffs break down under the frost quite rapidly and 

 heavy block taluses occur which are evidently postglacial ; but in 

 most cases there is little or no talus, specially outside the Potsdam 

 rocks. In the case of the limestone walls solution might be suffi- 

 cient agency to remove the products of weathering, and this might 

 also apply to the Precambric Grenville limestones which form some 

 part of the basins of the Redwood lakes ; but such removal can 

 not apply to the almost imperishable Potsdam sandstone. The 

 older fragmental deposits produced by the recession of the cliffs 

 have been removed, most likely by the glacial ice, but without 

 leaving much drift in their place. 



The lack of drift in the basins and over the plains clearly implies 

 a lack of drift burden in the latest ice sheet. The cause of this 

 will be discussed later [page 172]. The small abrading power of 

 the ice was probably due to its lack of tools, and evidently it did 

 not have sufficient power of " plucking " or removing blocks in 

 mass to destroy or even seriously cut the steep ledges and scarps 

 which stood across its path. 



One suggestion in partial explanation of the somewhat contradic- 

 tory features, is that stagnant ice occupied the strong depressions 

 ever which the upper ice moved by shearing. This would fairly 

 account for the absence of heavy drift in the basins and valleys 

 and the protection of the walls. Another suggestion takes account 

 of the fact that when the latest ice sheet disappeared from this 



