HISTORY OF THE COASTAL GRAVELS. 407 



"brought in sediments and dropped them in the lake. If sedimentation 

 proceeded at about the same rate as the enlargement of the lake, there 

 would never be a space between the central bar of gravel and the ice walls 

 wide enough to permit the formation of a delta, but the finer ddbris would 

 be carried away. The outlets of the lakes were too narrow to permit the 

 deposition of sediment within the tunnel until another enlargement or tlie 

 sea was reached. It does not seem probable that the surface waters couhl 

 take down beneath the ice heat enough to produce the larger glacial lakes. 

 In such cases we nuist postulate lakes open to the aii- and absorbing heat 

 from the sun. 



It is not meant to imply that in all cases the gravels were deposited in 

 the central parts of the lakes. The essential part of the process is that the 

 size and velocity of the streams bear such a ratio to the size of the lake 

 that the streams are not sufficiently checked to permit their depositing the 

 finer debris. Elsewhere are described the gravels of northeastern Mon- 

 mouth, where the glacial river flowed swiftly across the middle of small 

 glacial lakes, depositing a terrace of coarse gravel on each side of its course 

 and leaving a central ravine to mark its channel. 



Among the possible causes of a small enlargement of the subglacial 

 tunnels south of the north ends of the bays or fiords of the coast (fiord 

 line) may be mentioned an increased rate of ice flow at that line. The 

 fiords continue for some miles beneath the ocean, as is shown by the Coast 

 and Geodetic Survey charts, but they are shallow, and on the whole the 

 sea floor is less uneven than the land, and the slope southward is somewhat 

 steeper than the average land slopes north of the fiord line. We seem, 

 then, to have a right to assume that, near the shore, after the ice had passed 

 the higher obstructing hills, it would have its rate of motion somewhat accel- 

 erated. Crevasses due to tension owing to the more rapid flow towai'd the 

 ice front would be here more abundant than northward, while those due to 

 inequalities of the land would be rather less abundant. On the whole, the 

 conditions probably favored the restriction of the subglacial channels, but 

 it would be difficult to place a quantitative estimate on this agency. 



We must also consider the possibility that the retreat of the ice may 

 have been at very unequal rates, and that the gravels formed near or not 

 far back from the ice front would be determined in part at least by these 

 "varying conditions of retreat. If so, we might find corresponding types of 



