﻿GEOLOGY OF THOUSAND ISLANDS REGION I47 



that the knobby surface of the crystallines is the immensely ancient 

 erosion surface of the Precambric land area, which had been buried 

 under Potsdam sediments and only recently uncovered. Ice erosion 

 seems to have had very little influence in shaping the surface, merely 

 rounding and smoothing the knobs. 



The major axes of the knobs are roughly parallel with the valley 

 and the ice movement, but the relation to the latter is mostly casual 

 and not genetic. The struck or northwest side commonly shows 

 more erosion, but frequently the difference is not evident. As a 

 rule the crystallines have not retained their striae and polish as well 

 as the Potsdam sandstones. 



Plains of erosion. The broad plains, either rock or rock floored, 

 are regarded as the product of long eras of atmospheric erosion 

 with later glacial planing and a finishing touch of wave smoothing. 

 They are found in districts where the sedimentary rocks are persist- 

 ent in considerable thickness so as to cover the Precambric and the 

 lower and irregular Potsdam. Broad tracts of this class consisting 

 of upper Potsdam occur south of Chippewa Bay and toward Alex- 

 andria Bay. Theresa dolomite forms the plain north of Chippewa 

 Bay and covers large areas on the parallel of Plessis and Clayton. 

 South of the parallel of Lafargeville the plains and plateaus are 

 limestones. 



The earlier ice sheets seem to have lifted or plucked away the 

 weathered and weak superficial layers of these stratified rocks down 

 to some firm, less jointed and more resistant bed; but the flatness 

 and smoothness of these level stretches is partly due to the latest 

 action, the leveling action of the shallowing waters. The glacial 

 drift is commonly thin on these plains and patches of bare rock are 

 very frequent, sometimes acres in extent, specially on the Potsdam. 

 A good example is seen at Plessis, which village was formerly called 

 " Flat Rock." On the highways rock frequently occurs in unex- 

 pected manner and often forms the wagon track for considerable 

 distance. Although glacial polish and striae occur frequently on the 

 Potsdam the majority of exposures have either lost their smooth- 

 ness or were never severely rubbed. On the other strata glaciated 

 surfaces are not common. 



'These plains have been trenched by stream erosion and many of 

 the valley walls are yet steep, those of Chaumont river for example. 

 The differential erosion of the several strata has produced scarps 

 or benches about the margins of the higher plains which are fre- 

 quently striking features of the landscapes and sometimes are per- 

 sistent for long distances. These have been described in a former 



