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NATURE STUDY REVIEW [9 :5— May, 1913 



Fig. 1. A mountain valley which has been made U- 

 shape by glaciation. Note the comparatively smooth 

 sides and bottom. 



What further proof can be found? Note that the valley floor and 

 sides (at least part way up) have been swept clean and that 

 nearly all sharp angles are gone. In other words the loose boul- 

 ders have been rounded and the solid rocks have been worn 

 smooth. Could this be done by any other agency than ice ? Now 

 take the student across the valley floor. What formed those 

 grooves and scratches (striae) on the solid rock: and on many of 

 the loose boulders? Would clear ice cut such grooves? He 

 learns here that the great erosive power of ice is due to the 

 boulders that it carries. A little questioning will elicit how the 

 glacier secured the boulders. As he ascends the valley he may 

 find its head to be like a great amphitheater, i. e., a cirque, in 

 the bottom of which the great glaciers of past ages joined and 

 passed down the valley. This will be true if the valley studied 

 is in the mountains where local glaciers have existed. How did 

 the glacier or glaciers make it amphitheater-shaped? How did 

 they make those basins in the solid rock floor of the cirque that 

 now contain beautiful lakes? 



From this little trip up the valley he has learned of U-shaped 

 valleys, smoothed rock surfaces, grooves and striae, the absence 



