330 GLACIAL GRAVELS OF MAINE. 



flow. A superficial stream falling down a crevasse could also whirl the 

 water at a considerable depth, if it was of large size. The glacial water- 

 falls are often many hundred feet high, and the water attains a high velocity 

 in falling. How deep beneath the water potholes could thus be formed is 

 uncertain. In any particular case we should, in order even to guess, have 

 to know the size of the streams and the thickness of ice. 



5. The existence of glacial potholes in places remote from any recog- 

 nizable glacial gravels is proof that not every glacial stream left sediments. 

 Only the larger masses of the drift of these streams have thus far been 

 mapped. The smaller masses are buried beneath the englacial (upper) till 

 or the marine clays of the coast region. So also are the potholes and ero- 

 sion channels excavated by the subglacial rivers in the solid rock. I have 

 not found any of the latter in Maine which are of geological importance, 

 but Professor Dana showed me one of this kind near New Haven. 



FORMATION OF KAMES AND OSARS. 



Ridges, domes, and plains rising 50 to 150 feet above the surrounding 

 till testify that a very large amount of work has been expended in bringing 

 so great masses together. They usually rise to a greater height and show 

 greater thickness than equal areas of till in the same regions. On the 

 average they are areas of unusual accumulation. They can not have been 

 derived from the local subglacial till supplemented by the englacial till 

 contained in a body of ice of such length and breadth as at the given place 

 deposited an area of till equal to that covered by the gravel. A supply must 

 have been brought from abroad. And since a large amount of the finer 

 detritus of the till is washed away in the process of making glacial gravel, 

 this foreign supply must have been large. Such local accumulations might 

 be caused in various ways. 



1. In case of the longer glacial rivers, flowing as they did up and over 

 hills, we might expect areas of till erosion on steep down slopes or near the 

 tops of passes, where the swift streams carried all before them, alternating 

 with areas of accumulation. In places all the till, both subglacial and 

 englacial, has disappeared, and often all but the coarsest of the water-rolled 

 matter. In other places (as at The Notch, Grarland), the osar river did not 

 succeed in eroding all the till over which it flowed. This erosion of the till 

 in the course of osar rivers sometimes took place along a definite channel 



