398 GLACIAL GRAVELS OF MAINE. 



differential subsidence of the land whereby the proximal extremity of the 

 ice-sheet is more depressed than the distal. This effect would be intensified 

 if it was accompanied by a corresponding rise of the sea to diminish the 

 surface gradient of the waters of the glacial streams. 



In addition to the varying velocities of current that favor sedimenta- 

 tion, we also must reckon with friction and the force of gravity. Thus 

 manifestly the slopes of the bed of a subglacial stream favor sedimentation 

 when the stream leaves a steeper down slope for one less steep or for an up 

 slope, since a greater comjDonent of the force of gravity is to be overcome. 



Thus far in our discussion we have assumed the ice to be stationary. 

 But one of the important works of glacial ice is to push forward the sedi- 

 ments that gather in subglacial tunnels. Thus, in the Kettle moraine of 

 Wisconsin there are many stones that have been very much rounded by 

 water. action and subsequently thrust forward and incorporated ' with the 

 other moi'ainal matter. The same phenomenon is seen at the larger termi- 

 nal moraines of the Rocky Mountains. When the rate of ice flow is rapid 

 and the larger part of the debris is superficial, all or nearly all the glacial 

 gravel is brought forward to the front of the ice, partly by the streams and 

 partly by ice pushing. A considerable part of the waterworn matter is left 

 as a part of the moraine in most cases. Indeed, it is difficult to account for 

 gravels being deposited in transverse tunnels, or in the transverse portions 

 of tunnels, without being pushed forward, while the ice remains deep and 

 the flow rapid. It is equally difficult to account for gravels being pushed 

 forward by the ice that were deposited in longitudinal tunnels of uniform 

 size. Occasional mounds occupying caves in the base of the ice might be 

 pushed forward. Such pushing would probably obliterate the stratification, 

 but the floods of the succeeding summer would restratify it, and at the last, 

 when the ice became sufficiently thin, it would no longer be able to thrust 

 it onward, but woidd be forced to flow over it, or at the most could only dis- 

 organize a portion of the mass on the stoss side. In like manner we can 

 account in part for the failure of the ice to push forward transverse gravels 

 at a time of stagnant thin ice and rather rapid rate of enlargement of the 

 subglacial channels. 



When we come to apply these general principles to the problem of the 

 coastal gravels of Maine, we note, first, that domes and cones of gravel up 

 to one-eighth or even one-fourth of a mile wide were left on the tops of low 



