ICE-SHEET EROSION IN NEW YORK 49 



The Scandinavian geologists have claimed immense erosion of their 

 highlands in order to account for the great volume of drift spread over 

 the German lowlands and other areas marginal to the ice body. The 

 probable exaggeration of the drift volume is accompanied by an under- 

 estimate of the amount of rotted material in the crystalline regions, due 

 to millions of years of preglacial decay. Such arguments are attractive 

 in affording scope for the imagination, but they deal with uncertain 

 elements and are very indefinite and inconclusive. 



The best illustration of the erosional effects of a great continental 

 glacier is found in the broad area of Canada, and the facts are clearly 

 stated in the following extract from a recent letter by Professor A. P. 

 Coleman : 



11 It seems to me probable that a vast ice sheet, like the great Labradorian glacier, 

 would erode comparatively little near its center, much more powerfully midway 

 from the central area toward the margin, and not at all at the margin. Your 

 carefully studied New York region is too near the edge to show much erosion. 

 The bottom of the sheet was clogged with coarse and fine debris by the time it 

 reached you, and was incompetent to do much erosion. The same is true north 

 of lake Ontario, as at Scarboro Heights, where the advancing Wisconsin ice pushed 

 up over the stratified interglacial delta deposits of the Laurentian river with little 

 effect, even over the upper stratified sand. . . . 



"In the Sudbury district, however, much more scouring has been done, and 

 very little moraine stuff and no preglacial weathered rock is to be found. All has 

 been pared down to the fresh rock. The surface is, however, probably not very 

 greatly different in relief from the preglacial one, since the harder bosses and 

 ridges are still strongly marked hills, well rounded toward the northeast, rougher 

 toward the southwest. Many of them have the moutonn6e form, especially the 

 granite hills. 



" The great morainic deposits of southern Ontario consist mainly of very fresh 

 materials, even decomposable basic rocks, such as olivine diabases, coming out of 

 the boulder clay in perfect freshness. Our boulder clay and morainic stuff do not 

 show any signs of weathering or oxidation of the geest. Perhaps all the surface 

 material was swept farther south across the lake. 



"Though the amount of plucking and of erosion of live rock in this region, 

 where I imagine ice erosion to have been most effective, is important, I do not 

 estimate the average amount of cutting down of the surface as very great, probably 

 considerably under 100 feet. We have numerous rock-basin lakes, but so far as I 

 have seen all are small and not very deep." 



The territory of New England and New York lies in the intermediate 

 and debatable ground between central wear and marginal deposition. 

 The practical absence of interglacial deposits in New York may be due, 

 theoretically, either to greater erosive power of the later ice or to con- 

 tinuity of the ice occupation. 



The high topographic relief of eastern New York is regarded as unfa- 

 vorable to glacial erosion, but the great depressions followed by the ice 



