DIRECTIONS OF GLACIAL EIVERS. 319 



large areas. In the time of its strength the ice-sheet could so far strangle 

 its rivers that only a little sediment was left in their channels, but the 

 sediment was poured out in front of the ice where the sea now is. But in 

 its decay, when the flow became sluggish and even the ground turned 

 against it and increasing quantities of solar heat were transmitted throug-h 

 the ice, the gravel was left far back of the ice front. In the Rocky 

 Mountains substantially all the glacial gravels were frontal or overwash 

 plains, and the same was true of large portions of the northwestern Interior. 

 In Maine the marine deltas and a large part of the reticulated kames were 

 deposited in front of the ice, also much of the valley drift; but in addition 

 to these there is a very great development of gravels that were deposited 

 within the area then covered by the ice. For the great enlargement of the 

 glacial stream channels we need invoke only the same causes that first 

 established them as tunnels. Mechanical erosion, melting by warmed 

 waters, and heat transmitted through the ice, are sufficient to do the work 

 when acting through thin ice whose motion was sluggish or in places 

 almost arrested, aided by the rising sea, the bodies of water lying to the 

 north of the hills, the increasing quantities of water warmed under the 

 sunlight either by the melting of the roofs of their tunnels or their being 

 forced up onto the ice by the clogging of their channels, etc. Toward the 

 last probably most of the water in the channels of the broad osars or osar 

 terraces Avas exposed to the sunlight. If the narrowness of the early osars 

 is remarkable, the broadness of the later ones is equally remarkable. 



These extraordinar}^ enlargements of the stream channels were made 

 in the last days of the ice at the place of enlargement, all the other condi- 

 tions being favorable. Mechanical erosion was active, but still more eff"ect- 

 ive was that insinuating, ever alert agent, heat, whose transformations within 

 the decaying ice-sheet were varied and powerful. 



DIRECTIONS OF GLACIAL RIVERS COMPARED WITH THE FLOW OF THE ICE. 



Our definite knowledge of the courses of the rivers of the ice-sheet is 

 derived from the sediments they have left behind them and the excavations 

 they made in the till and the solid rock. When we map the gravels, we 

 map only those portions of their channels in which sediment was deposited. 

 In large portions of their courses the flow must have been too swift to per- 

 mit the deposition of sediment. While it is impossible now to reconstruct 



