200 THE PHENOMENA OF DRIFT, OR 



from sixty-eight to eighty-six feet ; and its elevation above the 

 adjacent surface, is from fifteen to tliirty feet. Its breadth is not 

 usually more than four or five rods through the base ; thougli this 

 last statement is not the result of actual measurement. On the 

 top it is frequently only wide enough for a good road. The 

 height of the East Ridge is not given in Mr. Gray's notes, though 

 I am confident it cannot be quite as high as the Indian Ridge. 

 The shape of these ridges, through a considerable part of their 

 course, is almost a semi-cylinder ; though usually somewhat flat- 

 tened, especially where the ridges interfere with one another. 

 The length of the East Ridge is one hundred and eighty-five 

 rods : of Indian Ridge, four hundred and twenty-three rods, or 

 one mile and one third : of West Ridge, five hundred and fifty- 

 nine rods, or one mile and three quarters. 



Now "what explanation shall we give of the origin of these 

 extraordinary ridges ? It seems to me hardly possible that any 

 one should examine them and not be satisfied that they must 

 have been the result of some other mechanical agency besides, or 

 in addition to, that of water. If we suppose large and successive 

 masses of ice to have been forced over the spot, filling up more 

 or less the valley of the Shawsheen, the lateral moraines which 

 they would pile up, would con-espond exactly almost to these 

 ridges. The great iiTcgularity of the rocky sides of the valley, 

 would force the mass of ice more or less out of its direct course, 

 to the right and the left, so as to account for the serpentine direc- 

 tion of the ridges. Successive icebergs, also, would often disturb 

 the moraines produced by those which had gone before, and 

 sometimes perhaps move the previous ones out of their place, 

 making to them the addition of a new moraine ; as seems to 

 have been done between A and C, by the agent that formed the 

 ridge CD. A similar interference may be seen in several other 

 places. If we suppose the West Ridge to have been produced 

 at first by a mass of ice, wide enough to fill the whole valley, the 

 Indian Ridge may have been subsequently formed by a smaller 

 mass, and the East Ridge by one still smaller. 



In still further confirmation of these views, it may be stated, 

 that the general com'se of these ridges corresponds very closely 



