I893.J y [Upham. 
ous or hilly country must gather much drift into its lower part, 
to as great height as the altitude of the mountains and hills, by 
grinding off and plucking away detritus and blocks of rock from 
these elevations, thence carrying them forward enclosed within 
the ice many hundreds of feet, and in the lee of the White, Green, 
and Adirondack Mountains even thousands of feet, above the 
ground. But it has seemed to some geologists difficult to account 
for the transportation of much drift into the ice from moderately 
undulating or plain districts, such as make the greater part of the 
drift-bearing areas of our continent and of Europe. On these 
nearly flat tracts, however, I find at localities in Minnesota and 
Manitoba good proofs, as they seem to me, that the thickness of 
the etiglacial drift was sometimes as much as forty feet near the 
ice-border where it was amassing prominent terminal moraines, 
and on lines or belts where confluent ice currents met from 
broad regions on each side.^ Similarly in England, according to 
the observations of Mr. G. W. Lamplugli, a confluent ice-sheet flow- 
ing from Scandinavia and Scotland was pushed up on the York- 
shire coast, bringing much englacial drift, with marine shells, which 
it had eroded and gathered up from the shallow and almost level 
basin occupied before the Ice age and again afterward by tlie 
North Sea.^ 
The manner in which the ice gathered drift into its basal por- 
tion from any plain tract seems to me explainable by a considera- 
tion of the currents of outflow toward its border. In the central 
area of the ice-sheet the currents of its upper and lower portions 
probably moved outward with nearly equal rates, the upper move- 
ment being slightly faster than at the base. Upon a belt extend- 
ing many miles back from the margin, however, where the slope of 
the ice surface had more descent, the upper currents of the ice, 
unsupported on the outer side, would move much faster than its 
lower currents which were impeded by friction on the land. 
There would be accordingly within this belt a strong tendency 
of the ice to flow outward with somewhat curved currents, 
tending first to cany the onwardly moving drift gradually 
upward into the ice-sheet, and later to bear it downward and 
' Geol. and nat. hist. surv. Minn. Ninth ann. rept., for 1880, p. 322-326. Final 
rept., vol. 1, 1884, p. 603, 604. Geol. and nat. hist. surv. Can. Ann. rept., new ser., 
vol. 4, for 1888-89, p. 38-40 E. 
2Qaart. joarn. geol. soj. L nil., vol. 47, 18c)l, p. 384-431, with maps and sections. 
