﻿GEOLOGY OF THOUSAND ISLANDS REGION 159 



The pitted clays are a link between the ice marginal deposits and 

 the open lake deposits. They might be classed with the morainal 

 or peripheral drift, since they were associated with remnants of the 

 ice front, but the aqueous origin is here regarded as the more im- 

 portant element. 



Glacial erosion 



General character. The abrasional work of the glacier in this 

 area is more conspicuous in the northern district where the hard 

 Precambric and Potsdam rocks are in high relief and the drift is 

 mostly in the hollows. Over the southern district where horizontal 

 limestones form the floor the ice erosion was probably greater than 

 farther north, but the evidences are more concealed. The origin 

 of the plains, plateaus and mesas, by preglacial weathering, glacial 

 planing and stream trenching, has been discussed in a former chapter 

 [p. 146]. 



The more vigorous erosion on the limestones is shown by flutings 

 or ribbing, the lighter and later, by striation and polish. The Pots- 

 dam and crystalline knobs seem to have been little more than " sand- 

 papered " by the latest glaciation. The broader surfaces of the 

 more horizontal Potsdam shows effective abrasion in spots only. 

 The impression made on the observer is that glaciation of an earlier 

 ice invasion was vigorous but that the latest ice sheet was compara- 

 tively ineffective. 1 



Striations. Occurrence. The limestones exhibit few striae, 

 as will be inferred from the lack of arrows on the maps of the 

 limestone districts [pi. 44-47]. It is uncertain whether this should 

 be chiefly attributed to the failure of the latest ice to generally abrade 

 the rock surface, due possibly to clayey character of the subglacial 

 drift in this district, or to the obliterative effect of solution and 

 weathering. The limestones are readily attacked by atmospheric 

 waters, as proven by the very numerous areas of solution structures 

 and open joints [p. 133, pi. 26-27, 35] • But in many places the 

 fresh removal of clay or clayey till that would seem to be sufficient 

 protection to the rock reveals unglaciated surface, though usually 

 firm and even, as if a glaciated surface had lost its smoothness. This 

 feature is emphasized by the finding in the same locality surfaces 



1 Unfortunately we have no standard or measure of- the intensity of ice 

 abrasion or erosion, or glaciation in general. When a writer says that the 

 drift is scanty or abundant, that erosion has been great or small, he ex- 

 presses merely his own conception of relative intensity, based on his obser- 

 vational experience. It is apparent that different observers might have 

 different opinions, according to the range of their work and their mental 

 attitude. Moreover, the view of thesame student might vary with increas- 

 ing experience and changing emphasis on the various elements or factors. 



