284 



GEOLOGY. 



beyond a certain point, the decrease of motion seriously interferes with 

 the efficiency of erosion. When any considerable thickness of ice at 

 the bottom of the glacier is full of debris, this loaded basal portion may 



Fig. 253. — Glacial striae and bruises. The block to the right shows two sets of striae: 

 that to the left shows the peculiar curved fractures known as Chatter Marks. 



Fig. 254. — Bowlders showing glacial striation. (Drawn by Miss Matz.) 



approach stagnancy, and the lower limit of considerable movement may 

 lie between the loaded ice below and the relatively clean ice above. A 

 moderate, but not an excessive load of debris is, therefore, favorable 



