LATE GLACIAL HISTORY OF THE COASTAL KEGION. 411 



large, as compared with the capacity of the tunnels, that they were able to 

 sweep their tunnels clear of sediments, or nearly so. In many places near 

 the coast there were formed at this period glacial lakes too large to be 

 ascribed to melting by subglacial waters and which wei'e probably open 

 above to the sun. The ice could have been only a few hundi-ed feet deep 

 at the time of their formation. They appear to have been formed by 

 gradual enlargement around a growing plain of gravel. Numerous marine 

 deltas are found in this region, sometimes alternating in the course of the 

 same gravel system with the massives or plains deposited in the glacial 

 lakes, which massives show little or none of the horizontal assortment of 

 sediments belonging to the delta deposited in still water. The deltas and 

 terminal moraines mark lines of retreat, but it is difficult to synchronize 

 them. 



SUMMARY. 



The waters of surface melting utilize the crevasses of the glacier for 

 penetrating to the bottom of the ice or into it, where they force a passage 

 along the crevasses or beneath the ice, assisted more or less by the basal 

 waters and furrows in the base of the ice. 



The narrow channels due to fracture or the crannies which the waters, 

 open by their pressure are enlarged by melting and mechanical erosion into 

 tunnels which sometimes expand into chambers, caves, and channels of 

 various shapes and sizes, and may open above to the sunlight. 



Other things being equal, when glaciers lie on the land and disappear 

 by melting without berg discharge, the amount of enlargement of the 

 tunnels varies directly as the time they are being enlarged, i. e., inversely 

 as the rate of ice motion. 



The enlargement of the tunnels is antagonized by a slow inward flow 

 of the ice walls. The laws that govern the rate of inward flow, how far 

 the rate is determined by the depth of ice or by variations of pressure 

 caused by the ice movement over obstacles or by heat transmitted through 

 the ice, etc., are unknown. 



The transfer of energy beneath the glacier by gently warmed surface 

 waters, the heat of which is generally available for the enlargement of the 

 subglacial or englacial tunnels by melting their walls, is greatly hindered 

 when the glacier flows into a body of water, since, as the warmed waters 

 pour into the cold waters that bathe the basal ice, they become more or less 



