48 H. L. FAIRCHILD — ICE EROSION THEORY A FALLACY 



normal product of atmospheric and stream work under some conditions 

 of vigorous drainage, with or without rock displacement.* 



Part II. Ice-sheet Erosion in New York 



GENERAL PRINCIPLES 



The previous writing (Part I) has treated specially of the work of alpine 

 or stream glaciers. These did not exist in New York — at least in the 

 part of the state which we will study. The phenomena here represent 

 the work of a " continental " ice body, and afford another critical test of 

 the doctrine of deep ice erosion — particularly as they include the 

 " Finger " Lake region. It needs to be clearly stated that any erosion 

 of the basins of the New York lakes was not by alpine glaciers, but by 

 mere lobations of the great Ontarian ice mass, as will be shown later. 



The work of continental ice sheets has not been so much in question 

 as that of stream glaciers; but the distinction often made between the 

 erosive effects of alpine and continental glaciers is not well founded. 

 Erosion is dependent on the combination of the factors of velocity, press- 

 ure, abrasive tools, and clearance, and any difference in the erosion by 

 stream glaciers or continental glaciers is a matter of the intensity and 

 reaction of these factors, the valid distinction being simply the variation 

 of these factors, the same as between two alpine glaciers. However, it 

 maybe granted that the action of continental glaciers may involve con- 

 ditions which can not be determined by study of stream glaciers. The 

 more general effect of continenal glaciers has been regarded as that of 

 leveling, by planing of elevations and rubbing of drift into the depres- 

 sion. Yet, while the extravagant claims for the erosive effects of the 

 " Polar ice-cap " made by the early glacialists have given place to more 

 moderate views, the idea is still prevalent that the ice bodies had great 

 excavating power. References are still made to " deepening of the Great 

 Lake basins" and to the " glacial origin of the Finger lakes." 



The general failure of continental glaciers to effectively erode even 

 soft deposits under their marginal portions is shown b}' the vertical suc- 

 cession of glacial and interglacial beds left over wide areas of slight relief, 

 as in the Mississippi area ; but a theoretical distinction has been drawn 

 'between peripheral zones of deposition and the central areas of erosion. 

 Naturally the evidences of removal of material and abrasion of surfaces 

 are pronounced in the centers of glaciation, and many observations are 

 recorded which indicate superficial erosion, but not deep cutting of firm 

 rock. 



* For an example of such features by faulting see "The hanging valleys of Georgetown, Colo- 

 rado," by W. O. Crosby, Tech. Quart., vol. xvi, March, 1903, pp. 41-50. 



