Pebeuart 21, 1913] 



SCIENCE 



297 



The average northward uplift or tilt of 

 the marine plane in the Hudson and Cham- 

 plain 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 up- 

 lift. It seems quite certain that the in- 

 crease of the gradient northward that is ap- 

 parent west of the Adirondacks must also 

 occur on the east of that mountain mass. 

 The differential uplift between the Iro- 

 quois plane at Rome (460 feet) and at 

 Covey Gulf, on the Canadian boundary 

 (1,025? feet), is about 565 in a distance of 

 149 miles in a direction 33° east of north, 

 giving a slant of 3.8 feet per mile. The 

 grade from Richland to East Watertown 

 is toward 6 feet per mile. 



In east and west direction there is small 

 deformation. The Iroquois plane at Ham- 

 ilton, Ont., is given as 363 feet. At Rome 

 it is 460 feet, which makes an eastward up- 

 lift of 100 feet in 225 miles, 0.4 foot 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 wave-like uplift 

 might be a fairly uniform plane, simula- 

 ting that produced by tilting of a rigid 

 surface. 



POSTGLACIAL EROSION 



Land erosion since the ice sheet disap- 

 peared is exhibited in wave cutting 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, Watkins glen. But 

 there are great numbers of glens or steep- 

 walled rock gorges throughout the state 

 which are quite as interesting and instruc- 

 tive as these, even if smaller and unadver- 

 tised. 



When applied to the effects of erosion in 

 New York the term "postglacial" needs 

 explanation, for much canyon cutting was 

 effected while the ice sheet still lingered on 

 territory of the state. For example, the 

 Portage ravine of the Genesee began cut- 

 ting while the ice front was not far away 

 on the north. The Mount Morris ravine, 

 the "High Banks," was in the making 

 while the ice covered Rochester. And the 

 upper (south) section of the Rochester 

 canyon was largely cut while Lake Iroquois 

 waters prevented the excavation of the 

 lower part of the gorge. Certainly a large 

 part of the erosional work in central and 

 western New York and the Hudson Valley 

 occurred while the glacier still covered the 

 northern lowlands of the state, including 

 the Champlain Valley. 



GLACIAL TIME 



The first question commonly asked by the 

 non-geologist is, "how long ago?" We 

 have to admit ignorance of any precise 

 measure of geologic time. Geologists have 

 learned to think in millions of years, and 

 they are not greatly concerned with the 

 precise duration of so short a period as the 

 glacial or postglacial episode. However, 

 precise knowledge is desirable and a yard- 

 stick of geologic time must be sought. All 

 attempts to use the present rate of canyon 

 cutting or cataract recession as an index of 

 time have failed, and no data yet discovered 

 have much value. 



The history of the ice-front recession 

 with its long succession of lakes and well- 



