GEOLOGIC EFFECTS OF THE ICE-SHEET 141 



This would seem to terminate the debate about glacial erosion in the 

 Finger Lake District. But it does not, as the responsibility for the 

 anomalous topography is shifted back to the pre-Wis'consin glaciers. 



"This carries with it the necessity of believing in 1,500 feet of vertical 

 erosion in the Seneca Valley by the continued ice-work of at least two periods 

 of glacial occupation, separated by an interval of gorge cutting several times 

 as long as the postglacial interval" (page 16). 



"The statement is warranted, therefore, that these valleys have been pro- 

 foundly modified by glacial erosion, both by deepening and broadening" (page 

 30). 



"But here, as in the process of glacial stream erosion, the bulk of the work 

 was done by an earlier ice advance" (page 31).' 



It is admitted that ice-sheets may have some individuality, and that 

 successive sheets on the same territory may have somewhat different be- 

 havior and produce different effects due to differences in the climatic, 

 topographic, and drift factors; but it does not seem reasonable that one 

 ice-sheet could deepen Seneca Valley 1,500 feet, while its successor did 

 practically no eroding at all. If the pre-Wisconsin ice-sheet had such 

 remarkable excavating power, it should have produced conspicuous ero- 

 sional effects elsewhere than in the valleys, and specially in the southern 

 part of the State, and should have piled heavy "old drift" deposits be- 

 yond the reach of the Wisconsin ice. 



The drift burden of the Laurentian ice-sheet is represented not merely 

 by the mass of the moraines and the volume of detritus carried away by 

 the glacial drainage, but also by the enormous bulk of drift built into 

 the drumlins. Even if the drumlins were partly constructed by the 

 earlier ice-sheet they can not, because of their location, represent any 

 product of deep erosion of the sections of Seneca and Cayuga valleys in 

 question. There are no heavy moraine deposits south of the Valley- 

 Heads moraine, for the terminal moraine is not massive, and the ancient 

 drift in Pennsylvania and New Jersey is not excessive in volume. The 

 only other disposal of the great volume of debris that should have been 

 produced by deepening of the valleys 1,500 feet must have been by out- 

 wash of the glacial drainage. But when I he valley-train and out wash 

 deposits attributed to the latest ice are considered there is no very large 

 volume left to represent any earlier drainage. 



The entire argument for deep ice erosion in the Finger Lake region 

 is based on physiographic features, hanging valleys and "oversteepened" 

 valley walls. The writer believes that sufficieni attention has not been 

 given to the effects of pre-Pleistocene drainage in connection with the 

 climatic, topographic, and diastrophic factors. The high elevation of 



