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PHYSICAL GEOLOGY 



" plucking" of the blocks of the rock than is removed from a soft 



limestone in which joints are poorly developed (p. 157). Under 



certain conditions glaciers may have 

 little effect on the underlying forma- 

 tions, as is shown by till and even 

 sand and clay deposits (Fig. 170) laid 

 down by an earlier ice sheet, which 

 were but slightly affected when over- 

 ridden by a later ice sheet. In 

 Switzerland glaciers have overridden 

 Alpine landslides without carrying 

 away many blocks. It is possible, 

 however, that such unconsolidated 

 deposits as those just described were 

 frozen when the ice moved over them. 

 Modification in the Shape of the 

 Hills. — The shape of the hills in 

 glaciated regions sometimes shows 

 the direction from which the ice 

 came, the side upon which the ice 

 impinged, called the stoss side, having 

 a more gentle slope than the other, 

 the lee side (Fig. 171). Hummocks 

 of rock eroded by glaciers and known 

 as roches moutonnees (p. 158) are 

 well-developed in many places, but 



may be especially well studied in portions of Canada. 



Effect of Glaciation on Drainage. — In general it can be said 



that glaciation tended to disturb the preexisting drainage, with the 

 »» > 



Fig. 170. — Till overlying lake 

 clays, showing that a lake first ex- 

 isted and that the ice sheet advanced 

 over the clays without being able 

 to remove them. Williamstown, 

 Massachusetts. 





FlG. 171. — Diagram showing the effect of glacial erosion. The stoss side surfers 

 more than the lee side, and the slopes are more gentle. The direction of ice movement 

 is shown by the arrow. 



result that land which in preglacial times was as thoroughly drained, 

 for example, as portions of West Virginia and Kentucky to-day, 

 became swampy, with abundant lakes and ponds. 



