160 H. L. FAIRCHlLD PLEISTOCENE GEOLOGY OF NEW YORK STATE 



in thickness and weight to zero in the region of New York Bay. The 

 amount of postglacial uplift increases from zero in the district of New 

 York Bay to over 700 feet on the north boundary of the State. The cor- 

 respondence between the thickness of the ice-cap and the amount of post- 

 glacial uplift of the land is very striking and significant. All about the 

 Laurentian Basin the tilted shores of the extinct glacial lakes afford us 

 evidence of the differential uplift of the glaciated territory. 



The average northward uplift or tilt of the marine plane in the Hud- 

 son and Champlain Valley appears to be about two and one-fourth feet 

 per mile, but some higher and as yet uncorrelated shore features in the 

 Champlain Valley suggest a deeper submergence there and a larger rate 

 of uplift. It seems quite certain that the increase of the gradient north- 

 ward that is apparent west of the Adirondacks must also occur on the 

 east of that mountain mass. The differential uplift between the Iroquois 

 plane at Eome (160 feet) and at Covey Gulf, on the Canadian boundary 

 (1,025 ? feet), is about 665 in a distance of 149 miles in a direction 33 

 degrees east of north, giving a slant of 3.8 feet per mile. The grade 

 from Eichland to East Watertown is toward 6 feet per mile. 



In east and west direction there is small deformation. The Iroquois 

 plane at Hamilton, Ontario, is given as 363 feet. At Eome it is 460 feet, 

 which makes an eastward uplift of 100 feet in 225 miles, 0.4 feet per 

 mile. 



The steadiness or uniformity of the tilted marine plane in the Hudson 

 and southern part of the Champlain valleys is somewhat surprising. It 

 does not seem probable that all land uplifting was deferred until the ice 

 was removed from a stretch of 200 miles, and that the rise and tilting 

 was that of a rigid mass. It would seem more likely that as the weight 

 of the ice-sheet was slowly removed it was followed by a progressive wave 

 of land uplift. However, the final result of an epeirogenic wavelike 

 uplift might be a fairly uniform plane, simulating that produced by tilt- 

 ing of a rigid surface. 



Postglacial Erosion 



Land erosion since the ice-sheet disappeared is exhibited in wave-cut- 

 ting by the lakes and canyon-cutting by diverted streams. In postglacial 

 ravines New York State excels. We may recall Niagara, the three ravines 

 in the course of the Genesee, the Ausable chasm, and Watkins Glen; 

 but there are great numbers of glens or steep-walled rock gorges through- 

 out the State which are quite as interesting and instructive as these, even 

 if smaller and unadvertised. 



When applied to the effects of erosion in New York, the term "post- 



