NONCONTINUODS SEDIMENTATION IN ICE CHANNELS. 397 



of the glacial and the frontal water) is in equilibrium. This part of the 

 tunnel and all its connecting crevasses are permanantly filled with water 

 that can flow only when there is an effective head of water rising above or 

 behind it. At the proximal end of this permanent body of water the 

 streams in summer flow with such velocity as to keep their channels clear 

 of sediment, but during the fall and winter the small streams are checked 

 as they flow into the body of permanent water and deposit their sediments, 

 probably to wash it away during the next flood. A rising or falling sea or 

 frontal lake might under some conditions cause the deposition of a series of 

 such sediments formed at the successive levels of the subglacial portions 

 of the sea or lake. 



Among the conditions of retreat in presence of a cold sea (the depres- 

 sion of the Chiegnecto Peninsula would assist in maintaining a rather cold 

 sea on the coast of Maine) is the marginal zone of submarine ice. When a 

 glacier ends in a warm body of water, the ice margin overhangs; but when 

 it ends in a cold body of water, and especially in salt water that can be 

 cooled below 32°, the waves erode the upper ice just as they do other 

 rocks, and leave cliffs overhanging- the surf, while there is left a shelf of ice 

 passing out beneath the sea and from time to time breaking off in blocks 

 and rising to the surface.^ The breadth of this zone of submarine ice 

 would be increased if the basal ice contained a considerable quantity of 

 glacial d(^bris and thus weighted it down, so as to prevent it from rising as 

 bergs. The breadth of this ice would increase during the winter, partly 

 because of the colder sea and partly on account of the violence of the 

 winter storms in eroding away the upper part of the ice. In summer the 

 ice would begin to melt this projecting shelf, and under favorable circum- 

 stances might before autumn melt it all away and even cause an overhang 

 of the upper ice. 



If crevasses opened from the subglacial tunnels upward to the surface 

 of this submarine ice, the subglacial streams would rise in the crevasses and 

 escape on the surface of the sea, partly because their specific gravity is less 

 than sea water and partly because they would thus meet less friction. If 

 so, the stream would drop much of its sediment at the base of the crevasse. 



Another cause of the diminished velocity of the subglacial streams is a 



'Eussell, Nat. Geog. Mag., vol. 3, p. 101, 1892. 



