56 CLYDE MAXWELL BAUER 



terrace have been mapped by the writer nearly to its headwaters. 

 A few miles west of Ekalaka, Montana, several flat-topped, gravel- 

 covered ridges 50 to 60 feet high form a prominent feature of the 

 ^landscape. The terrace, where noted, is covered with gravel from 

 a few inches to 10 or 12 feet in thickness. The gravel consists 

 principally of red and white quartzite pebbles as well as some of 

 chert and argillite, ranging from the size of sand grains to 2 or 3 

 inches in diameter. The terrace has its best development along 

 the Little Missouri in southwestern North Dakota, and at this 

 place it probably represents the late Tertiary valley floor of the 

 Little Missouri, which existed until the invasion of the earlier ice 

 sheet. Since then the Little Missouri has cut a channel to the east 

 and shortened its course. The present trough of the Little Missouri, 

 which is about 200 feet deep near Medora, is therefore the result of 

 erosion since the disappearance of the early ice sheet. 



Parts of the Missouri River above Poplar, Montana, and like- 

 wise the Yellowstone River, probably had their beginning at the end 

 of the Cretaceous period, when the seas withdrew and exposed large 

 areas in the region of the Rocky Mountains to stream erosion. 

 These primitive streams and their associates were apparently 

 small at first, but as the land area grew by withdrawal of the sea 

 and with the rising of the mountain belt, in the early part of the 

 Eocene epoch they became larger, gained velocity, and gathered 

 sediment which was gradually spread out over the region now 

 known as the great Fort Union area. These rivers doubtless 

 meandered widely over this broad palustral flat and had their 

 embouchure somewhere to the north," probably into the predecessor 

 of Hudson Bay or the Arctic Ocean. 



As the primitive YeUowstone and Upper Missouri and their 

 tributaries and allies continued their work, several thousand feet 

 of sediment were deposited during the Eocene epoch in eastern 

 Montana, western North Dakota, and parts of adjoining states. 

 This was finaUy interrupted by an epeirogenic movement, which 



' W. G. Tight, Abstract of paper read before the Geological Society of America, 

 Geol. Soc. Amer. Bull, XVIII. 



Warren Upham, "Age of Missouri River," American Geologist, XXXIV (1904) 

 80-87. 



