﻿GEOLOGY OF THOUSAND ISLANDS REGION 171 



with apparently identical physical conditions have lost all traces 

 of recent glaciation. 



Weak erosion of the Wisconsin ice sheet. It will be seen that 

 the critical point in this study is the erosional impotence of the 

 latest ice sheet. With this established then at least dual glacia- 

 tion of the region must be accepted. 



The principle is recognized by glacialists that intensity of ice 

 erosion depends on pressure, velocity of the bottom ice, and its 

 armament or tools. The glacier can do its most effective work of 

 abrasion when the basal' ice is only moderately charged with rock 

 rubbish, and that of hard texture. A heavy burden of subglacial 

 drift serves to diminish the plasticity of the ice and so reduce the 

 velocity of flow; while at the same time it acts as protection or a 

 buffer for the subadjacent rock. For this reason rapid corrasion is 

 a self-checking process. 1 On the other hand it is certain that clear 

 ice can not abrade the bed rock at all. A moderate load of hard 

 tools is the most effective for abrasion. 



The first ice sheet that transgressed our region found it deeply 

 covered with the residual product of millions of years of weather- 

 ing, and could do no effective erosion until not only the sheet of 

 geest (regolith) on our area had been removed but also that lying 

 on the region northward into Labrador and Canada which was 

 swept by the southward ice flow. The theoretical stages would 

 be as follows : ( i ) the scraping away of the decay product and 

 bearing it far southward, as no very heavy moraines lie near our 

 area; (2) vigorous erosion during the phase of favorable load, 

 with harder tools from the plucking of the fresher rocks; (3) 

 weak abrasion by the clearer ice after the glacier had swept its 

 floor and reduced the asperities in its path. 



On the postulate of a single ice invasion of the Thousand Islands 

 region it is necessary to assume that after the ice had removed 

 the abundant product of Prepleistocene weathering it used its 

 medium load of debris to plane the hard Potsdam and to plane and 

 deeply flute the limestone, but at the same time failed to rub down 

 the scores of comparatively abrupt cliffs and scarps which opposed 

 its motion. Here we find another inconsistency. Without further 

 discussion it will be understood that the assumption of a single 

 glacial epoch involves serious contradictions and difficulties in the 

 explanation of the phenomena of the region. 



Assuming dual or multiple glacial epochs the features and history 

 are fairly clear. The accumulations of long eras of rock weathering 



iGeol. Soc. Am. Bui. 16:26. 



