﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 69 



the hollow being produced partly by the washing of the clay cover 

 down the wide solution joints. On the sides of the hollow the ribs 

 are being newly uncovered by the storm wash and the tramping 

 of cattle, but no trace of glacial polish was seen, the new surfaces 

 being similar to the longer exposed surfaces of the middle of the 

 ribs. Masses of chert standing 2 or 3 inches above the limestone 

 surface prove a long period of solution of the rock surface, which 

 seems impossible beneath the present clays in the short time in- 

 volved. Enforcing this conclusion is the fact that only a number 

 of rods distant, in the gutters of the electric road under similar clay 

 cover the same limestone shows elegant glaciation. But while the 

 ribbing is s. 40 w. the preserved glacial scorings are variable, rang- 

 ing from s. 50 w. to s. io° e. 



These two examples of' the ribbing, which can be multiplied, will 

 give illustration of the quality of the evidence they offer in favor 

 of at least dual glaciation in recent time. These flutings are wide- 

 spread, remarkably uniform in direction (generally s. 42-45 w.), 

 symmetrical and true in form. They can not be attributed to 

 weathering, nor jointing, nor wave work, nor water corrosion, all 

 of which have left conspicuous records in the district. Undoubtedly 

 the ribbing is old glacial, and it represents a glacial abrasion vastly 

 more energetic than the similar work of the latest ice sheet. 



Weathered surfaces. The considerable weathering which the 

 limestones have suffered is shown in plates 23, 26, 2.J and 63. 

 Doubtless some part of this corrosion is postglacial, specially on 

 the more exposed patches and on cliff edges where the rock was 

 not buried by the drift; but it must not be assumed that these 

 etched, corroded and open-jointed surfaces were all left smooth 

 and glaciated by the latest ice sheet, as that is the question under 

 discussion. 



The uncovering of corroded surfaces which have been under 

 clay that would seem to have been sufficient protection from the 

 postglacial weathering, as illustrated in plates 61 and 62, argues 

 strongly for nonglaciation of such surfaces. The conditions shown 

 in plate 60 seems to prove that great open joints existed in the 

 limestones previous to the deposition of the glacial clays. 



Probably many of the deeply corroded surfaces were recently 

 buried under ice or lake deposits which have been swept off by 

 the wave erosion of the standing waters and the subsequent and 

 now acting storm wash. Without more special study on the ground 

 it is impossible to estimate the amount of postglacial weathering. 

 In some places it seems very small, and where slightly covered prac- 



