TESTS OP SUBGLACIAL OE STJPEEFIOIAL DEPOSITION. 423 



ANGLE OF LATERAL SLOPE OF THE EIDGBS. 



The lateral slopes of the ridges are in general rather less steep in the 

 region below than in that above 230 feet. Not only the lenticular kames 

 but also the continuous ridges have as a rule rounded summits and gentle 

 side slopes below 230 feet. This is partly, but not wholly, due to the 

 waves of the sea washing over the tops of the ridges. Assuming that the 

 lenticular eskers were formed beneath the ice and that their gentle side 

 slopes are in part due to the action of the ice in flowing over them, we can 

 not set up that fact as a crucial test for subglacial streams. The overhang- 

 ing walls of a superficial stream may impinge on the contained gravels, and 

 when these channels were greatly enlarged at the base, the contained 

 gravels might have as gentle slopes as the subglacial. In the interior of 

 the State some of the ridges have very steep lateral slopes, and are of 

 uneven size, and show hummocky heaps like a terminal moraine. I do not 

 see how we can admit that the ice flowed over these ridges since their com- 

 pletion. If they are stratified at their bases, they must have been deposited 

 in superficial channels, the gravel rising above the basal enlargements or, 

 in subglacial tunnels after the ice had ceased to flow, or nearly so. The 

 test here is not infallible, but the probabilities slightly favor the supei-ficial 

 streams. 



INTERNAL STRUCTURE. 



Sediments deposited beneath the ice must be stratified unless the strati- 

 fication is obliterated by the pushing forward of the sediments by the ice. 

 Facts are elsewhere recorded indicating that the ice had a limited power to 

 disorganize small portions of eskers on their stoss sides. In various places 

 the osars appear to have lost their stratification. At one time I thought the 

 Corinna-Dixmont osar had been disorganized where it crossed valleys, while 

 it remained stratified on the hills. Later excavations make this doubtful. 

 It is very difficult to find excavations in Maine that do not show more or 

 less surface sliding, unless they have been made very recently. Seldom 

 can a sand-and-gravel bed be implicitly trusted after even a single winter. 

 I therefore leave out of account many cases of apparently pellmell struc- 

 ture observed in the earlier years of my exploration, since my notes do 

 not definitely show that the excavations had been made during the summer 

 they were examined. A residue remains where osars have apparently no 



