396 GLACIAL GEAVELS OF MAINE. 



conditions independent of the presence of a body of water rising above 

 the bottom of the ice at its distal extremity. 



Moving waters drop a portion of their load of sediments when their 

 velocity is for any reason diminished, or they have a greater component of 

 the force of gravity to overcome, provided thej were just able to carrj^ the 

 load. 



One cause of a reduced velocity of current is the enlargement of the 

 channel. Local enlargements of the tunnel of a subglacial stream, result- 

 ing in a localized slowing of the waters, are formed at the bases of cre- 

 vasses down which warmed superficial waters pour. They may also form 

 in rapids where the waters rebound u^pward in passing over rocks, or where 

 they fall to the ground again and spread laterally, or where the subglacial 

 waters rise into crevasses or onto the surface because of insufficient or 

 clogged outlets. Perhaps the most important method of enlarging narrow 

 tunnels into lake basins is that whereby a large superficial stream forms a 

 pool or lake where it pours down its shaft into a subglacial or englacial 

 tunnel. Grradually the Avarmed surface waters melt a large shaft and ulti- 

 mately form a pool. If from time to time the outlet became clogged or 

 proved insufficient, so that the pool and shaft became filled with water 

 exposed at the surface to the sunlight, the melting would be accelerated 

 and ultimately a lake would be formed. When once the water of the lake 

 became exposed to the sun for a large part of the time, the enlargement 

 would be still more rapid. A lake might also form where, on account of 

 stoppage of the subglacial stream, the waters rose through crevasses onto 

 the ice and absorbed heat from the sun The extension of a subglacial 

 stream northward incidental to the thinning of the ice might cause a series 

 of new crevasses to open across the course of a superficial stream and a 

 corresponding series of enlargements at their bases. In these and other 

 ways we can account for local enlargements of subglacial tunnels into hol- 

 low cones, domes, and caves of various shapes, also into basins open above 

 to the air, the impetuous waters acting on the ice both by mechanical 

 erosion and by melting. 



When a subglacial stream tunnel passes up and over a hill or opens at 

 the ice front beneath a body of water, some special phenomena demand 

 notice. The water in the tunnel below the top of the hill or surface of the 

 body of water (making allowance for the difference in the specific gravity 



