Qo NEW YORK STATE MUSEUM 
New York is literally strewn with thousands of glacial boulders or 
erratics which were transported from the Adirondacks by the ice, 
and similar boulders are occasionally found as far south as Bing- 
hamton or well down the Hudson valley. Evidences of glaciation 
also occur high up in the Adirondacks and the Catskills, so that the 
greatest depth of ice over New York State could not have been 
less than several thousand feet. In fact, we have every reason to 
believe that the Adirondacks, if not the Catskills, were completely 
buried. The reader may wonder how an ice sheet a mile thick in 
northern New York could have thinned out to disappearance at or 
near the southern border of the State, but observations on existing 
glaciers show that it is quite the habit of extensive ice bodies to 
thin out very rapidly near the margins, thus producing steep slopes 
along the ice fronts. 
Successive ice invasions. The front of the great ice sheet, like 
that of ordinary valley glaciers, must have shown many advances 
and retreats. In the northern Mississippi valley, however, we have 
positive proof for several (perhaps five or six) important advances 
and retreats of the ice which gave rise to true interglacial stages. 
The strongest evidence is the presence of successive layers of glacial 
debris, a given layer often having been oxidized, eroded, and cov- 
ered with vegetation before the next (overlying) layer was 
deposited. In drilling wells through the glacial deposits of lowa, 
for example, two distinct deposits or layers of vegetation are often 
encountered at depths of from 100 to 200 feet. Near Toronto, 
Canada, plants which actually belong much farther south in a 
warmer climate have been found between two layers of glacial 
debris. Thus we know that some, at least, of the ice retreats pro- 
duced interglacial stages with warmer climate and were sufficient 
greatly to reduce the size of the continental ice sheet or possibly 
to cause its entire disappearance. ; 
In New York State no very positive evidence has as yet been 
found to prove truly multiple glaciation, though some phenomena 
as, for example, certain buried gorges, are very difficult to account 
for except on the basis of more than one advance and retreat of 
the ice. At any rate, there appears to be no good reason whatever 
to believe that there were more than two advances and retreats of 
the ice over the State, and for our purpose in considering only the 
general effects of glaciation we may practically disregard the prob- 
lem of multiple glaciation because the final effects would have been 
essentially the same as a result of a single great glacial advance and 
retreat. 
