Crlfi'i-tii of Ihuff. — Uphaui. 3X1 
Daring the Cliamplaiu epoch, :is the time of disappearance of the 
last ice-sheet has been named by Dana, its snperficial melting was 
rapid throughout the warm portion of each jear. while the sul)- 
ghicial melting went on at a very slow rate through both winter 
and summer, the same as it had been during the entire epoch of 
ghiciation. Owing to the rapidity of the melting on the ice sur- 
face, and to tlie amount of englacial drift thus exposed and sul»- 
jected to erosion and transportation, we believe that the subglacial 
stream c(jurses already existing were inade([uate for the drainage, 
and that they were mostly obstructed and closed by the transpor- 
tation and deposition of modified drift. The waning ice-fields 
were then deeply incised by brooks and rivers j)ouring o\er them 
in the descent to their border and to the adjacent hind lately un- 
covered by the glacial retreat. Hydrograpliic basins of the ice- 
sheet probably extended 50 to 200 miles or more from its margin, 
resembling those of a belt of country along a sea coast: but the 
glacial rivers, and their large and small hranclies. had much 
steeper gradients than those of the present river systems on the 
land surface, and often or generall}- they flowed in deep ice- 
Avalled channels, more like canons than ordinary river valle3s. 
Much englacial drift, which had become superglacial. Avas washed 
away b}' the rains, rills, and small and large streams from the ice 
surface; and the osar gravel ridges are the coarsest sediments 
])rogressively deposited near the ice-front in such channels which 
were cut backward into the retreating edge of the ice liy the su- 
perglacial streams. 
The best development of osars on this continent, scarcely in- 
ferior to that of Sweden, is found in Maine and has Iteen thor- 
oughly explored and studied by Prof, (ieorge 11. Stone, who con- 
cludes that the material forming these long ridges, al.so the short 
ridges and knolls caHed kames. and the valley drift, or stratified 
gravel, sand. clay, and tine silt, spread along the rivci' courses 
and on the iowl;iiids. were all supplied chiefly fi'oin the iMiglacial 
drift.* This origin sccnis to me also true for the kames. osars. 
and valley drift which have conu' under niy ol(ser\ation in N'ew 
Ifampshire. A'ermont and Massachusetts, and in Miuui'sota. ^lani- 
loba, and adjacent portions of the Northwest. In a papei- read 
* Proceedings of the Boston Society of Natural History, vol. xx, 1880, 
pp. 430-409. Proc., A. A. A. S., vol. xxix, 1880, pp. .5lO-51!l. .\m. .bmr. 
Sci., Ill, vol. xl, I8n0, pp. 1'2-M44. 
