SOME INSTANCES OF MODERATE GLACIAL EROSION 1 



RALPH S. TARR 



Cornell University 



Instances of marked glacial erosion have in recent years been 

 reported from many sections, but very little has been written regard- 

 ing evidences of moderate glacial erosion. In several widely sepa- 

 rated localities I have found definite proof of moderate ice-erosion, 

 which, in view of the growing tendency to assign to ice great erosive 

 power, seems worthy of statement at the present time. These 

 instances will be considered one by one. 



SOUTHERN CENTRAL NEW YORK 



South of the Finger Lakes is a plateau upland which was com- 

 pletely overridden by ice of the Wisconsin stage. Across this upland 

 extends a series of moraines marking a prolonged halt of the receding 

 ice. South of the recessional morainic belt there are numerous 

 evidences of moderate glacial erosion, and no proof that the topog- 

 raphy was perceptibly modified by ice-erosion, although during the 

 time that the ice-front stood at the terminal moraine, about 50 miles 

 farther south in Pennsylvania, the highest hills were completely 

 covered. 



The most important evidence of moderate glacial erosion in this 

 plateau region is / supplied by the presence, in numerous localities, of 

 residually decayed materials in place, not only on lee slopes, but on 

 hilltops and on the stoss sides of hills. The decayed material varies 

 from discolored and disintegrated shale fragments beneath sandstone 

 caps to fine-grained residual clay resulting from the decay of the shale 

 (Fig. 1). In such cases the sandstone cap layers are cracked and 

 broken, and the decayed shale is in some instances three feet below 



1 This paper was presented before the first meeting of the Association of American 

 Geographers at Philadelphia. The facts relating to New York were discovered while 

 working for the U. S. Geological Survey, and are published by permission of the 

 director. 



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