THE GEOLOGICAL HISTORY OF NEW YORK STATE QI 
Ice erosion. Ice, like lowing water, has very little erosive effect 
upon rocks unless it is properly supplied with tools. When flowing 
ice is shod with hard rock fragments the power to erode is often 
pronounced because the work of abrasion is mostly accomplished 
by the rock fragments rather than by the soft ice itself. For in- 
stance, when the great ice lobe moved up the St Lawrence valley it 
was shod with many pieces of hard Precambric rocks, and the effects 
of erosion are remarkably well shown in the Thousand Islands 
region. Thus, about two miles due south of Clayton the writer 
has seen a succession of great grooves, covering an area of several 
acres, and cut into the hard, fresh Potsdam sandstone on top of a 
low hill. A little search will reveal polished and scratched or 
grooved rock surfaces in almost any part of the State. Granite 
ledges in the Adirondacks are often glaciated, and the freshness 
and hardness of the surface rock proves that the ice eroded all the 
deep preglacial soil as well as the zone of rotten rock, and an 
unknown amount of live or fresh rock. ; 
In former years a very great erosive power was ascribed to flow- 
ing ice, but today some glacialists consider ice erosion to be almost 
negligible, while many others maintain that, under favorable con- 
ditions, flowing ice has a very considerable erosive effect. During 
the very long preglacial time, rock decomposition must have pro- 
gressed so far that rotten rock, including soils, had accumulated to 
considerable depths, as today in the southern states. Such soils are 
called “residual” because they are derived by the decomposition 
of the very rocks on which they rest. But now one rarely ever 
sees rotten rock or soil in its original place in New York because 
such materials were nearly all scoured off by the passage of the 
great ice sheet, mixed with other soils and ground up rock frag- 
ments and deposited elsewhere. Such are called transported soils. 
Along the southern side of the State, where the erosive power of 
the ice was least, rotten rock is not so uncommonly seen. 
Ice, shod with hard rock fragments and flowing through a deep, 
comparatively narrow valley of soft rock, is especially powerful as 
an erosive agent because the abrasive tools are supplied; the work 
to be done is easy; and the increased depth of the ice where 
crowded into a deep, narrow valley causes greater pressure on the 
bottom and sides of the valley. Many of the valleys of northern 
New York were thus very favorably situated for ice erosion, as 
for example, the Champlain, St Lawrence, Black river, and Finger 
lakes valleys, as well as many of the nearly north-south valleys of 
the Adirondacks, The writer has made a special study of ice 
