Uphara.] 10 [Nov. i6, 
deposit it parti}- l)eneatli the edge of the ice and partly along the 
ice boundary. 
Whenever the warm climate terminating the Glacial period 
extended unchecked through many years, the depth of the ablation 
or superficial mehiug of the outer part of the ice-sheet was proba- 
bly not less than from 15 to 25 feet each summer, as has been 
observed on the Muir glacier in Alaska^ and on the Mer de Glace 
in Switzerland.^ At such rates of melting any district enveloped 
by ice from '2,000 to 4,000 feet thick, as was true of the central 
portions of New England and doubtless also of a broad belt 
thence west to the Laurentian lakes and to Minnesota and south- 
ern Manitoba, would be uncovered in one or two centuries, and 
the recession of the glacial boundary Avould average probably a 
half mile or more yearly. 
During any long series of years when the ice-sheet was being 
thus rapidly melted, its outer portion to a distance of probably 
twenty miles from its boundary, being reduced by ablation to a 
thickness ranging from 100 feet and less upward to 1,000 feet, 
would bear on its surface, especially in the valleys and hydro- 
graphic basins of its melting, much drift which had been before 
contained in the higher part of the ice. Only scanty englacial 
drift, mainly consisting of boulders borne away from hills and 
mountains, appears to have existed at altitudes exceeding 1,000 
or 1,500 feet; but all the lower ice probably contained an increas- 
ing proportion of detritus and boulders which had been brought 
into it from below by the upward movements due to faster flow 
of the central and upper glacial currents than of those retarded by 
friction on the ground. The thinned border of the ice-sheet 
upon the belt having a remaining thickness of less than 1,000 feet 
would therefore become covered with drift, as Russell has 
described the borders of the Malaspina glacier or ice-sheet, which 
stretches from the Mt. St. Elias range to the ocean. '^ 
At many times the general recession of the ice-sheet Avas tem- 
porarily interrupted. The return of a prevailingly cold climate 
for several decades of years, or occasionally, as we may suppose, 
' H. F. Reid, in Nat.geof^r.niag., vol. 4, March, 1892, p. 31, 38. 
' Prestwich's Geology, vol. 1, p. 176. 
3 Nat.geogr. mag., vol. 3, 18'Jl, p. 53-203, with 19 plates and maps. Ainer. joiirn. 
pci., ser. 3, vol. 43, March, 1892, p. 169-182, with map. Amer. geol., vol. 8, Dec, 
1891, p. 384. 
