GLACIAL PHENOMENA IN MAINE. 
151 
coast, afford means of estimating approximate¬ 
ly the thickness of the ice-sheet which once 
moved over the whole land, as well as its limi¬ 
tations during a later period, when it had be¬ 
gun to wane. In order to advance across a 
hilly country and over mountainous ridges ris¬ 
ing to a height of twelve and fifteen hundred 
feet in the southern part of the State, and to 
a much higher level in its northern portion, 
the ice must have been several times thicker 
than the height of the inequalities over which 
it passed; otherwise it would have become 
encased between these elevations, which would 
have acted as walls to enclose it. We are 
therefore justified in supposing that the ice¬ 
fields, when they poured from the north over 
New England to the sea, had a thickness of at 
least five or six. thousand feet. On a future 
occasion I shall give an account of the drift 
phenomena along our Atlantic coast, showing 
also that at that period the ice-fields were not 
bounded by our present shore line, but extend¬ 
ed considerably beyond it, over surfaces now 
occupied by the ocean. At a later time, dur¬ 
ing the shrinking and gradual disappearance 
of the ice-sheet, the ice, no doubt, retreated 
