1908] HARVEY—PRAIRIE-GRASS FORMATION 83 
features of the Missouri valley drainage. This movement particu- 
larly affected this region by elevating it from an estuary condition 
to a point considerably above sea-level, and even probably many feet 
above its present level. This uplift inaugurated a period of vast 
erosion, and before the advent of the ice the Missouri had cut its 
present great valley at least 20 to 25™ below its present level. 
With the Pleistocene came the glaciers. While doubtless there 
Were five periods of glacial advance and recession in the region, we 
need concern ourselves now only with the second, the Kansan, which 
spread from the Keewatin center and deposited over this entire region 
the Kansan drift sheet, obliterating the questionable pre-Kansan. 
The Illinois, Iowan, and Wisconsin epochs followed successively, 
only the latter reaching into southern South Dakota, where a lobe 
of the Altamont moraine pushed down between the Big Sioux and 
Missouri Rivers, reachin g approximately to Vermillion, South Dakota. 
The Kansan must have seriously interfered with the established pre- 
glacial drainage, greatly rejuvenating it, at least along minor lines. 
5s seems probable also that subsequent erosion mainly sought out 
Previous lines, largely reestablishing the post-Pliocene drainage 
system. The Wisconsin likewise disturbed and caused a readjust- 
ment of this drainage system, which could have differed but little 
from that previously worked out in the Kansan. Upon this read- 
Justment of the post-glacial Wisconsin drainage topography, there 
followed the deposition of that much mooted deposit, the loess. 
The region divides itself naturally into two great topographic 
id the rolling upland prairie and the flood plain, which cuts the 
Prainle in a general northwest and southeast direction. On either 
= the flood plain is limited by the escarpments of the Missouri. 
aa flood plain extend the minor flood plains of its tributaries, 
. aes ‘ssecting the upland. The vast valley is cut out from 25 
Wario: — in the upland and presents a flood plain varying from a 
sei ‘trace to frequently ro™ in width. 
tiie ee now presents an almost perfectly developed erosion 
quent} gel predetermined in the Kansas drift sheet and subse- 
tter A Spite by the Wisconsin drift and loess deposit. The 
consin sie frequently has a depth of so™ to the south of the Wis- 
area, thins out northward. The escarpment bluffs are 
