The continental ice-sheet produced important drainage changes in western North Dakota. Its effects are particularly well shown in the case of the Missouri, the Yellowstone, and the Little Missouri rivers, since all these streams were forced to seek new channels. The region was three times invaded by the ice-sheet—the later Wisconsin, earlier Wisconsin, and an earlier invasion, which was probably Kansan or possibly sub-Aftonian—but it was the earlier, or pre-Wisconsin, invasion which caused most of the changes. The southerly course of the Missouri River below old Fort Stevenson has been attributed to the latest or later Wisconsin ice-sheet, but evidence is here presented that the valley, at least in North Dakota, is preglacial, using the term preglacial to mean older than the oldest ice-invasion of this region—it may mean either pre-Kansan or presub-Aftonian.

Age of the Missouri River Valley

As long ago as 1868 Gen. G. K. Warren made . . .

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