A PARTICULAR CASE OF GLACIAL EROSION 355 



The two chief features in this case were, I think, formed under 

 the following circumstances: At an early stage, glacial erosion cut 

 the stratified bed-rock unevenly, so that, when deposited, the till, G T 

 rested at the left on the clay-shales, Sc (now shoved over to the right), 

 in the middle on the limestone with shaly seams Sb, and toward the 

 right on the limestones Sa and B 2 . The later ice-movement was over 

 the till G x . Its friction with the till produced stress, which was 

 transmitted to the stratified rocks, and they yielded along the lowest 

 clay-shale seam, moving forward. The initial disjoining of the 

 stratum (Sb) and the till (G) x , may be ascribed to unequal stress 

 from unequal friction of the glacier on its bed. After the gap began 



i ■ — ^-» 2 



-- "' Q.--^ 



Fig. 2. — Diagram showing change of direction of ice-movement from 1-2 to 3-2 

 as the mass M moves from L toward O. Original position of ice and subjacent 

 rocks in solid line; change by movement, in broken lines. 



to open, the ice settled into it, while the rock-mass was moving 

 forward. The upward thrust in front and the gap at the rear may be 

 correlated through a compensating movement in the glacial ice., 

 as illustrated in Fig. 2. In this diagram the original position of 

 the ice, the older drift, and the subjacent stratified rocks are repre- 

 sented in solid lines, while the result of the movement is indicated 

 by broken fines and dots. Ice filled into the gap, L, as the drifted 

 mass M tended to fill the position at O, in its stead. The horizontal 

 line (1-2) indicates the original direction, while the other (3-2) 

 indicates the subsequent direction, and the angle (1-2-3) tne relative 

 rotation of the ice-mass in compensating. 



Whether this work of the glacier was done far from or near to 

 the front of the ice-sheet does not appear. The reason why the 

 drifted mass stopped where it is now seen is also not evident, and 

 may be a very incidental matter. Notably, however, the manne 



