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 tlie 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 gi'eat 

 amount of wearing was brought about at the source of the trail 1 It is, 

 liowever, 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 debris which it conveyed. As before remai'ked, there 

 are reasons to suspect thiit 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, I 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. 



