TESTS 'OF SUBGLAGIAL OR SUPERFICIAL DEPOSITION, 429 



tunnel' suffices to cany off the water without its rising in the crevasses, the 

 velocity is chiefly determined by the land slopes, but any surplus causes 

 some of the water to rise in the crevasses as into the standpipes of an 

 aqueduct system. The only limit to the effective "head" in the crevasses is 

 determined by the height of the tops of the crevasses over which the water 

 can overflow on the surface. During the summer floods, when the supply 

 of water is large as compared with the capacity of the tunnels, the water 

 may often be driven by the pressure of hundreds of feet of water in the 

 tunnel and crevasses. In other words, the effective "head" of subglacial 

 streams can not exceed the vertical differences in height between the mouth 

 of the tunnel and the top of the nearest crevasse connecting with the tunnel, 

 and therefore subject to overflow. When we come to compare the two kinds 

 of stream with respect to velocity, we find a mechanism in both cases for 

 producing high velocities with corresponding coarseness of sediments. It 

 is doubtful whether we are able to distinguish between the two kinds of 

 stream by the size of separate fragments of the sediments. 



EROSION OF THE GROUND MORAINE. 



Both kinds of stream would erode tlie subglacial till while in contact 

 with it. A subglacial stream being necessarily in contact with the lower till 

 the whole time of its flow, ought to erode it more than a superficial stream, 

 which could reach it only after it had cut its way to the bottom of the ice. 



Erosion beneath the osars. — Tlfis Is a difficult subject of iuvestigatiou, owing to 

 the character of the exposures. Artificial excavations do not go deep 

 enough, and at the rivers which have eroded the osars there is almost 

 always surface sliding of the gravel from above. At Pittsfield Village the 

 Sebasticook Eiver has eroded one side of the Hartland-Montville osar and 

 the gravel distinctly lies upon the bare rock. At numerous places the 

 gravel near the edge of the osar overlies the till, but this may be due in 

 part to surface sliding since deposition. At Clinton and various other places 

 excavations show that the gravel near the axis of the ridge extends nearly 

 to the rock, and then the base of the gravel was not reached. The facts 

 observed are too few for generalization, but point to considerable erosion of 

 the ground moraine beneath the osars. 



Erosion of the ground moraine in places not now covered by gravel. AloUg the COUrSCS of 



the osar rivers are many gaps in the ridges where we can now see the 



