No. 11. — The Conditions of Erosion beneath deep Glaciers, hased 

 upon a Study of the Boiddcr Train from Iron Hill, Cumber- 

 land, R. I. By N. S. Shalek. 



The conditions which determine the erosive action effected by conti- 

 nental ice sheets are as yet imperfectly understood. "We are not able 

 to penetrate beneath the existing accumulations of this type ; their 

 position makes even their superficial phenomena matters of difficult 

 inquiry. All that we can hope to ascertain concerning the work done 

 beneath these massive glaciers must be inferred from the eftects which 

 they have exercised upon the surfaces over which they once moved and 

 from which they have passed away. 



Certain of these effects are so clearly indicated that there is no diffi- 

 culty in interpreting the actions to which they were due. Others as 

 yet remain extremely obscure. As the following inquiry was under- 

 taken with the hope of clearing up some of these obscure problems, I 

 shall preface this essay by a brief statement as to the present condition 

 of our knowledge concerning certain features of the mechanical work 

 done beneath the ice during the Glacial Period. The following described 

 points may be regarded as fairly well established, viz. : — 



1. That the continental glaciers of the last Ice Period, though they 

 clearly moved forward in the direction of the ice front, did not always, 

 or even generally, accumulate large bodies of morainal matter at their 

 margins ; the frontal moraines being on the whole small in amount as 

 compared with the evident depth of the ice, and the distance over which 

 the materials were moved. 



2. That wherever the rocks over which the ice moved were of a na- 

 ture to furnish hard fragments, these were plentifully removed, and the 

 consequent erosion of the surface went forward in a tolerably rapid 

 manner. 



3. That the fragments loosened from the bed rock by the action of 

 the glacier were often borne to a considerable distance from their points 

 of origin, and that in this journeying the amount of erosion to which 

 they were subjected varied greatly, some of the erratics evidently re- 

 maining in contact with the bed rock, and serving to score or abrade its 



VOL. XVI. — NO. 11. 1 



