Accii III Illation of Dm mJi iix. — f j>/i(iiii. 355 
Manitoba, would l)e uncovered in one or two centuries, and the 
recession of the glacial Ixxmdnrv would average prolialily :i half 
mile or more yearly. 
During any long series of years when the ice-sheet was lieing 
thus rapidly melted, its outer portion to a distance of prol)ably 
twenty miles from its boundary, being reduced liy ablation to a 
thickness ranging from 100 feet and less upward to 1,000 feet, 
would bear on its surface. esi>ecially 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 l)Oulders borne away from hills and 
mountains, api)ears 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 1)g1ow l)y 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 Kussell has de- 
scribed the borders of the Malaspina glacier ov ice-sheet, which 
stretches from the Mt. St. Elias range to the ocean.* 
Sfriifiiiii iif Sii ixiijliii-idl Drift tM)i<h' nii<i'ni Kiuilncnil In/ iii- 
irriisii/ Siioir/d// (I ml hii Ai/vtnti-c nf fin- fhicl.i r jxirtnni of the hi- 
sliri t. — At many times the general recession of the ice-sheet was 
temporarily interrupted. The return of a prevailingly cold cli- 
mate for several decades of years, or occasionally, as we may 
suppose, for a century or more, l)rought increased snowfall, which 
sutficed to hold the ice boundary nearly stationarj-, perhaps fre- 
({uently first having pushed it again a considerable distance for- 
ward. The thick ice lying far back from the border may then 
have flowed over its previously' thin and drift-covered outer belt, 
aiding with the new snowfall to enveloi)e the once superglacial 
drift stratum within the ice-sheet. These halts or re-advances, if 
the front of the ice had a nearly constant position during several 
years, became marked by terminal moraines, of which T have 
mapped a series of eleven in consecutive order from south to 
north in Minnesota, North Dakota, and Manitoba, while Mr. 
^National (Teof^raphic INIngazine, vol. iir, 1891, pp. 53-208, wit li lit 
plates and niap.s. Am. .Four. Sci., HI, vol. xmfi, pj). 1()5)-I82, with inu]). 
March, !S9l>. .\m. ( ;i:..i.c)(;ist, vol. viii. |). :«4, Dec. iS!)l. 
