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, lowan, 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, reaching 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. 

 It 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 readjustment 

 of this drainage system, which could have differed but little from 

 that previously worked out in the Kansan. Upon this readjust- 

 ment 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 

 types, the rolling upland prairie and the flood plain, which cuts the 

 prairie in a general northwest and southeast direction. On either 

 side the flood plain is limited by the escarpments of the Missouri. 

 From this flood plain extended the minor flood plains of its tributaries, 

 further dissecting the upland. The vast valley is cut out from 25 

 to 50"^ deep in the upland and presents a flood plain varying from a 

 narrow terrace to frequently lo^'" in width. 



The upland now presents an almost perfectly developed erosion 

 topography, predetermined in the Kansas drift sheet and subse- 

 quently veneered by the Wisconsin drift and loess deposit. The 

 latter, which frequently has a depth of 50™ to the south of the Wis- 

 consin drift area, thins out northward. The escarpment bluffs are 



