Topography and the distribution of gravels support suggestions by earlier investigators that the master streams of northeastern Montana and northwestern North Dakota flowed northward across the present trench of the Missouri until diverted in the Pleistocene. The age of the pre-diversion drainage is not precisely known. The ancestral streams cross early Tertiary structures. It has been suggested that the streams are antecedent and are pre-Tertiary. However, middle or later Tertiary superposition from a high-level erosion surface seems equally possible.

Miocene-Pliocene (?) gravels along the ancestral valleys indicate that the streams date back at least to the latter part of the Tertiary. A high-level gravel of possibly Oligocene or Miocene age parallels Yellowstone Valley for long distances, suggesting that the Yellowstone may have followed approximately its present path across the area in mid-Tertiary time.

The ancestral Missouri, Yellowstone, and Little Missouri rivers were diverted to form the present Missouri probably in Kansan time. This is earlier than the date proposed for the glacial Missouri in South Dakota, but the dates are not necessarily contradictory. Little Missouri River was diverted again by early Wisconsin ice and joins the Missouri 80 miles farther downstream than it did formerly.

Both the Yellowstone and Missouri hug the right sides of their valleys and apparently did so in pre-Pleistocene time. Uptilting to the northwest or the effects of terrestrial rotation are possible explanations. Further work is required to test these possibilities.

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