﻿I/O NEW YORK STATE MUSEUM 



tically nothing. Such cases give the impression of slight corrosion 

 since the ice removal. On the other hand the existence of broad 

 surfaces of exceedingly rough and open-jointed rock, from which 

 the farmers have to fence their cattle, and the location of which 

 would seem to have been favorable to glaciation, give the suggestion 

 of large postglacial weathering. The critical question is, were the 

 latter surfaces glaciated by the latest ice sheet? It would appear 

 that a duration and intensity of postglacial weathering which has 

 not destroyed the glacial polish on the limestone ribs shown in 

 plate 61 could not justly be held responsible for the open joints 

 and rough surfaces shown in plate 60, where a deep clay cover has 

 been removed chiefly by washing down into the open joints and 

 being carried away by subterranean flow. 



The amount of recent weathering is conspicuously greater in 

 locations where the surfaces have been subjected to wave wash of 

 the Iroquois and Gilbert waters, this being specially effective in 

 both solution and mechanical removal of the limestone. 



Old planation surfaces. If the ribbing on the limestone was 

 in existence before the last ice invasion then, of course, the lime- 

 stone plains and plateaus were formed previously; and it has al- 

 ready been stated that the broader topographic features are con- 

 fidently believed to antedate Wisconsin glaciation. An ice sheet 

 with sufficient vigor to do the plucking and planing necessary to 

 give the limestones and Potsdam sandstones their breadth of flat- 

 ness should leave abundant evidence in glaciated surfaces; and the 

 limestone ribbing is a relic of such effective erosion. Again, the 

 general lack of glacial polish is the fact which requires explanation. 



The plains of Potsdam sandstone present the same question. 

 Over broad surfaces of the very firm, hard, insoluble sandstone, 

 either bare or practically unprotected by any impervious cover, only 

 a minor part exhibits striae or polish. Certainly it was once all 

 vigorously glaciated, for in no other way could the level, even, firm 

 surfaces be produced. From hasty examination it is impossible to 

 confidently decide whether the patchy scoring and polishing is due 

 to weak recent glaciation on an old weathered surface, or to reten- 

 tion of polish under present weathering. The former alternative, 

 recent partial smoothing on an old weathered surface, is more in 

 harmony with the general body of fact; and it seems more prob- 

 able that the recent ice sheet failed to generally polish the old 

 weathered surface than that patches with finest polish and hairline 

 striae should be so perfectly preserved while surrounding surfaces 



