208 BULLETIN OF THE 
boulders, — at least during the closing stages of the Ice Period. By far 
the greater part of the material was removed in the condition of fine 
sand, which has been to a great extent swept away from the path of the 
trail. Accepting this estimate, we have to reckon the erosion of this 
surface during the period when the trail between the hill and Provi- 
dence was formed as amounting to about three hundred feet in depth. 
As the distance between Iron Hill and Providence is about seventy-five 
thousand feet, the question arises, Can we assume that, during the pas- 
sage of the ice along this length of its course, anything like this great 
amount of wearing was brought about at the source of the trail? It is, 
however, by no means certain that the distance traversed by the ice 
during the formation of the trail did not exceed the length of the field 
occupied by the débris which it conveyed. As before remarked, there 
are reasons to suspect that the ice advanced and retreated several times 
while it lay on this part of the shore-land, and these advances and re- 
treats may have materially prolonged the time during which the ice 
continued to move over the surface of the hill. If the ice long retained 
a stationary front at any point between Providence and Iron Hill, or if 
its margin were subjected to successive oscillations, at no time falling 
back to the north of Iron Hill, then the boulders on this field represent 
the wearing effected by the passage of a much greater length of ice 
sheet than is indicated by the longitudinal extent of this part of the 
trail. 
After careful examination, I am inclined to doubt whether any con- 
siderable irregularities of movement such as have just been suggested 
ever occurred in this part of the glacier while the train was forming. I 
can find no trace of frontal moraines, such as would have been caused 
by any considerable pause in the retreat of the ice or the re-advance of 
its frontal wall. Therefore, while granting the probability of a certain 
amount of oscillation in a glacial margin, [ am not disposed to think 
that these accidents could have been of such magnitude as entirely to 
invalidate the computations as to the rate of erosion which we have just 
made. 
I have before noted the probability that the ice went off from this 
district, not by the gradual retreat of its front to the northward, nor by 
stagnation followed by a slow process of melting, but by the floating 
away of the thinned glacier in the waters of the sea, which at the close 
of the ice time stood at a higher level than at present. If the ice sheet 
thus departed in the form of bergs, we may the more readily account for 
the prevailing absence of small frontal moraines which we might expect 
to mark the stages of its retreat. 
