430 GLACIAL GRAVELS OF MAINE. 



former beds of these rivers. In a few places, as northwest of North Mon- 

 mouth and northeast of Hogback Mountain in Montville, a ravine of erosion 

 has been excavated in the till. Grenerally where the larger glacial rivers 

 crossed the hills, or on steep down slopes, Ave do not find a definite raAdne 

 of erosion, but the till is scanty or almost wholly absent over an area several 

 times as broad as the ordinary breadth of the osar. In these places there is 

 less till than in the surrounding country, and we must admit a large removal 

 of till, both the englacial and the subglacial. On the other hand, there 

 has been but little erosion of till in several passes and on several divides 

 where the circumstances would appear to be favorable to erosion. Among 

 these jnay be named the pass south of Grand Lake on the Houlton system, 

 the divide near Forest station on the Hersey-Danforth branch, the Katahdin 

 system in a low pass situated just northwest of the Whalesback in Aurora, 

 The Notch in Garland, and the valley of the east branch of Georges River 

 in Montville. 



We have, then, several cases of very great erosion of the till on the 

 line of the osar rivers, many cases where there has been a moderate erosion, 

 and perhaps an equal number of cases where there are now no gravels yet 

 there has been but little erosion of the till by large osar rivers. No posi- 

 tive inferences can as yet be drawn from the observed facts bearing on the 

 question of subglacial versus superglacial streams, though the probabilities 

 rather favor the superficial streams. On the theory of subglacial streams 

 it is difficult to account for such facts as are elsewhere recorded as being- 

 observed at The Notch in Garland. While there are a large number of 

 cases where the subglacial hypothesis is equally in accord with the facts, 

 and in some cases better in accord with them than the hypothesis of super- 

 ficial streams, there are other places where superficial streams are as 

 strongly indicated b)'' the facts. All this points to the conclusion that the 

 osar rivers were in some places subglacial and in other places superficial or 

 englacial. This may be bad for the symmetry of theories, but seems to be 

 true to nature. 



GAPS IN THE OSARS. 



Both subglacial and superglacial streams could sweep their channels 

 free from sediment at places where the channel was narrower or shallower, 

 or where the slopes of the land gave unusual velocity to the current. The 

 velocity of subglacial streams is certainly often much greater than that due 



