A PARTICULAR CASE OF GLACIAL EROSION 



in broken lines was exposed by quarrying, while the rest was con- 

 cealed. This part, represented in broken lines, is drawn from 

 memory. 



The part of the section shown in the right half of the profile was 

 exposed several years later, and showed that the glacial movement 

 was from the westward, not from the eastward or southeastward, 

 as was originally supposed. Only during the past year has the left 

 half, as represented in the profile, been cut across in such ways as 

 to reveal the two drift deposits, G x and G 2 , as well as the undisturbed 

 part of the stratum Sb. 



It is now evident that the newer till, marked G 2 , represents the 

 position of the ice which forced the stratum Sb apart, and shoved 

 the one part, Sb, with its overlying drift, G 1} from X to X x . At the 

 same time, the surface between Y and Y x was scratched, though 

 not much worn. The glacial deposit G T is older than G 2 , though 

 the two are in contact. They are essentially alike, though the pro- 

 portion of pebbles is greater in one than in the other, and the boundary 

 between them is structurally well marked. 



The mass of shale and limestone which was moved forward rests 

 on the surface of a stratum which, from Y t to X 3 , is not glaciated 

 or cut, being protected by a clay lamina, while from A r 3 to X' 3 the 

 surface below the shoved block is glaciated, and one thin layer, for 

 the greater part of the distance, has been ground away. In the 

 undisturbed strata at the left (northwest) there is a clay-shale lamina 

 at the base of Sb, and it is the lowest of several such partings between 

 the limestone layers. The disturbed mass started to move upon the 

 lowest shaly lamina, and was driven out over a denuded or already 

 glaciated surface X 3 to X f 3 . 



All the glaciated surfaces, excepting that which is indicated by 

 Fto Yj, were originally one, running from Aon the left, along X t , to 

 X' ' j, which was originally continuous with A 2 and down along 

 A'' 3 , as we see if we imagine the disturbed strata put back in place. 

 Then with the second till, G 2 , eliminated, we obtain the conditions 

 which preceded the movement. In this it will be noted, further, that 

 the interval or gap, from X to X x , on the left, winch should represent 

 the distance traveled, is less than the distance from X 2 to A" 2 plus 

 that from A^ 3 to X' 3 , which should also represent the distance traveled. 



