154 The American Geologist. Septembei-, i9U4. 
the ice-sheet when it was being finally melted away. East from 
the Scioto river in Ohio, and westward from the upper Mis- 
souri across the great Cordilleran region, the later drift and 
its moraines reach generally quite to the margin of glaciation, 
covering the older drift of the previous stages. 
One of these stages, named the lowan, has been shown 
by McGee in northeastern Iowa, and elsewhere by Leverett 
and many other observers, to have been characterized by ex- 
tensive deposition of loess, a fine silt washed down from the 
dissolving and waning icefields, and deposited by the flooded 
streams in front of the lowan drift boundaries, with much 
redistribution at the same time and later upon large tracts 
by winds. The loess evidently belongs to a time of general 
depression of the land beneath the long continued weight of 
the ice-sheet; and this change, from a former high elevation 
to mostly lower altitudes than now, appears to have brought 
again the warm temperate climate under which the continental 
ice-sheet from then onward was somewhat rapidly melted 
away, meanwhile forming many marginal moraines at lines of 
slackening, halt, or temporary readvance, in its general re- 
treat. 
On the plains bordering the upper Missouri river in the 
neighborhood of Fort Benton, and thence northward, loess, or 
a loesslike fine silt, is described by Weed as spread very exten- 
sively, \:arying from a few feet to a hundred feet in thickness, 
mantling the nearly flat country, and forming the upper part 
of the drift there.* Much further exploration is desirable for 
all the upper Missouri region, to discriminate and map the re- 
spective areas of the Albertan, Kansan, and lowan drift, and to 
trace the limits of the Wisconsin drift, lying farther north, 
with its moraines, in Assiniboia and Alberta, to the Rocky 
mountains. 
The oldest glacial drift sheet yet recognized, the Albertan, 
was so named by Dr. George M. Dawson from its occurrence 
on the upper edge of the plains of Alberta, along the east base 
of the mountains, and westward among the Rocky mountain 
ranges, whence its materials are wholly derived.! The Al- 
bertan till and associated Saskatchewan gravels have therefore 
* Fort Benton Folio, No. 55, Oeologic Atlas of the L'nited States, 1S99. 
\ Journal of Geology, vol. iii, pp. 507-511, July-.4.ugust, 1895; Bulletin, 
Geol. Soc. of America, vol. vii, pp. 31-ti6, Nov., lt>95. 
