TESTS OF SUBGLACIAL OE SUPERFICIAL DEPOSITION. 435 



type, the streams iu places having- velocity sufficient to clear their channels 

 of sediments. 



Later, when the ice became thin and could no longer flow up the hill, 

 this stagnant condition would favor the enlargement of the tunnels in spite 

 of the interference of the basal waters. When the ice surface sank to the 

 top of the transverse hill, or near to it, the stream could no longer escape 

 southward over the hill. It would then escape transversely to the east or 

 west along the top of the ice or between tlie ice front and the hill, or by 

 transverse subglacial channels. But in most cases the rivers crossed the 

 hills by passes leading up to low cols, and the hills at the sides of these 

 valleys would hold in the stream till the ice had melted back to the north 

 ends of the passes. The retreat of the ice from the tops of the divides back 

 to the northern ends of the passes might occupy several or even many years, 

 and during all this time there would be a marginal bodj^ of water between 

 the ice and the top of the col, absorbing heat direct from the sunlight. This 

 water would most rapidly extend itself northward along the line of the sub- 

 glacial river, partly through mechanical erosion and the Iieat of the stream 

 waters and partly because the ice near the tunnel would already have become 

 somewhat honeycombed by melting within the crevasses above the tunnel. 

 Thus the frontal lake would be narrowly V-shaped, extending deeply into 

 the ice, as an enlargement of the original tunnel, expanding toward the 

 south till it passed beyond the ice front and extended across the whole 

 valley or pass. Into this deltoid body of water the glacial river poured its 

 sediments. The coarser matter was left near the mouth of the subglacial 

 tunnel, and thence the sediments woixld grow finer obliquely outward. If 

 the lake became very broad as compared with the size of the rivei', we might 

 even have a delta deposited in it like that in Unity and Thorndike, or in 

 Dover, northwest of The Notch, Garland. If so narrow that the velocity 

 of the current was less checked, an osar terrace or broad osar would be 

 "deposited in the marginal lake, like the terraees that border the Whalesback 

 in Woodstock, Milton, and Rumford. In the lake or within narrower ice chan- 

 nels near its northern end, a plexus of reticulated ridges might be deposited. 

 The development of these broad-channel or lacustrine sediments would go 

 on retreatally northward till the ice front receded to the north ends of the 

 passes, when the waters might or might not be diverted into new courses 

 back of the ice front, but in any case the surface of the marginal lake began 



