PAPERS OX GEOGRAPHY AXD GEOLOGY 195 



trated in this area. The highest peaks have been glaci- 

 ated, indicating that the ice was at least 1200 feet in alti- 

 tude above the present level of Lake Huron. The low- 

 lands are polished and stripped to the bed rock in most 

 places, so that the granite plain i> hunmioeky with the 

 roches montonnees so characteristic of glaciated coun- 

 tries, with small swamps and ponds in the intervening 

 depressions. 



Along the lake shores the minor effects of ice erosion 

 are well exposed. Deep grooving both above and below 

 water level is strikingly developed on the northern shores 



■me of those islands which are composed of d 

 fine grained granite gniess. (Figure 1.) This sort of 

 abrasion on a la greatly modified the shape 



of the shore line, especially in such places as the south 

 shore of Philip Edward Island, and the mainland and 

 fringing islands east of Beaverstone Bay. where the 

 structure of the rocks is parallel to the direction of the 

 ice movement. In many places small and giant chatter 

 marks are common. 



Chatter marks are concave towards the direction from 

 which the ice came, and they are on the stoss side of the 

 roches montonnees. There are also certain break- which 

 are the opposite of chatter marks; they are on the lee side 

 of the rocks, and they are concave toward the direction 

 in which the ice was going. They are not true features 

 of plucking because they do not follow apparent joint 

 planes, nor are they controlled by them. They appear 

 to have been formed much in the same way as giant chat- 

 ter marks, except that they are on the upper part of the 

 lee side of the rocks, and that they are concave in the 

 opposite direction to that characteristic of chatter marks. 

 They break away on the lee side apparently because there 

 is a breaking face on that side. The reason that chatter 

 marks are scoop shaped is because there is no vertical 

 breaking face, and the plane of fracture returns to the 

 surface, which is the only available breaking face. 



The granitic rocks are pitted with shallow depressions 

 in many places after the fa.-hion of pot-holes. 3 

 2.) Pot-holes are usually thought of as those depres- 

 sions which are worn by swirling boulders or pebbles 



