BOWLDERS OF THE GLACIAL GEAVELS. 333 



thickest; yet it is also possible that the presence of abuudaut subglacial 

 waters facilitates the flow of ice in some degree. Both causes would pro- 

 duce an oblique flow inward toward the line of swiftest motion. Such a 

 movement would bring till matter to the stream, or nearer to it. 



So far as movements of the ice brought the matter of the kames and 

 osars together, they distinctly resemble the medial, lateral, or terminal 

 moraines of ordinary valley glaciers. Converging glacial strise are else- 

 where recorded. 



BOWLDERS OF THE GLACIAL GRAVELS. 



When bowlders are found on the surface of masses of glacial sedi- 

 ments, it is important to determine whether they have been worn and pol- 

 ished by water action. This often requires considerable excavation, since 

 it is only beneath the earth, where it has been protected from the action of 

 the weather, that we can expect the polished surface to have been preserved. 

 Many bowlders of coarse granite have so far weathered and fallen to pieces 

 since the glacial epoch that even beneath the ground it is now impossible 

 to know with certainty whether they were once polished or not. Omitting, 

 then, some undetermined cases, the bowlders of the glacial sediments may 

 be classed as follows: 



1. Below the highest level of the sea are many bowlders not smoothed 

 by ruiming water which overlie both the fossiliferous marine clays and the 

 coarser glacial sediments, also the osar border clays. They have the shapes 

 and rough surfaces characteristic of bowlders of the upper (englacial) till. 

 They are scattered here and there at intervals, generally one in a place, but 

 sometimes, especially on the north sides of hills, in heaps and sheets. They 

 are most abundant on slopes favorable to the grounding of floes of shore ice. 

 The deposits are so discontinuous and helter-skelter in their distribution and 

 so unlike a sheet of till in composition and structure that I attribute them 

 to floes of shore ice or small bergs. Two theories suggest themselves: 

 that there was a readvance of the ice over the marine clays, the ice con- 

 taining but little drift, or that the bowlders tumbled down from the ice upon 

 clays formed in front of the ice during its retreat before the sea. 



2. Bowlders are sometimes found in the till beneath the glacial gravel 

 and projecting upward into the gravel. In some cases the parts projecting 

 above the till were distinctly water-polished. This is a very common 

 occurrence at the marine glacial deltas. 



