TESTS OF SUBGLACIAL OR SUPERFICIAL DEPOSITION. 433 



PHENOMENA OF GLACIAL RIVERS IN GROSSING HILLS AND VALLEYS. 



As before noted, the hills of Maine are in large part transverse to the 

 direction of glaciation. Hence the courses of the longer glacial rivers very 

 often led them up and over hills. On the steeper down slopes the behavior 

 of subglacial and superglacial streams would, perhaps, not be very unlike, 

 but in the valleys and on the northern slopes of hills their action might 

 be quite different. The osars are in the main stratified, and the only 

 superglacial streams here referred to are those the bottoms of whose can- 

 yons had reached the ground at the time of deposition of the gravel, or so 

 nearly that the stratification was only obliterated locally, if at all, during 

 the unequal melting of the subjacent ice. This I conceive could take place 

 orAj in the marginal region near the ice front. Some distance back from 

 the front a superglacial channel might contain sediment, if the englacial 

 dtibris reached so high as the ice, but it would overlie such deep ice that 

 if left in this condition the unequal melting of the subjacent ice would 

 usually confuse the stratification. It is not assumed, except for the sake 

 of argument, that such streams helped to deposit the osars. 



We first suppose a subglacial tunnel to cross a transverse valley and 

 hill, as in the accompanying diagram, fig. 32. The water in the tunnel 

 below the horizontal line ^ 



AB touching the top of 

 the hill will form a dam 

 or lakelet and be in equi- 



llDllUm, like tne water j-ig, 32.— Icleal section of glaclal stream channels crossins trausverae valleya. 

 of a sewer tran Water -EBO, glacier; ^B-O.subglacial stream ;£, C, transverse Inlls. 



will rise to the same level in all crevasses opening into the tunnel. As fast 

 as water flows from the north into the trap an equal amount will flow 

 southward over the hill at B. The general law of velocities in the tunnels 

 is that if the tunnels increase in capacity from north to south at an equal 

 ratio with the increasing supply of drainage waters, other things being 

 equal, the velocities will be uniform throughout the whole courses of the 

 tunnels. But there are at least two causes for the tunnel being smaller in 

 the valley than elsewhere — that is, relatively to the supply of glacial waters. 

 First. The depth of ice, and presumably the rate of inward flow of the 

 tunnel walls, is greater in the valley at B. 

 MON xxxiv 28 



