54 CLYDE MAXWELL BAUER 



Small lakes are common along the old channel, and in the vicinity 

 of Coalridge the valley trough is very distinct. A. L. Beekley 

 traced this old valley from Medicine Lake to the North Dakota 

 boundary in 1910, and describes its features in Bulletin 471, U.S. 

 Geological Survey. 



From the Missouri Coteau northeastward the ancient stream 

 course is unknown, but it seems probable that it joined the pre- 

 glacial Souris River and flowed thence into the predecessor of 

 Assiniboine River, thence northward by way of Lake Winnipeg and 

 Nelson River to Hudson Bay. 



Elevations above sea along this course, some of which are shown 

 on the map, point to this conclusion. Most of the altitudes given 

 are of the lowest point in the immediate locality. If we take these 

 altitudes in order, beginning at Poplar, Montana, and continu- 

 ing northeastward, we find that, with the exception of Medicine 

 Lake, which is 1,970, there is a gradual decrease in altitude from 

 1,958 feet on the present Missouri River bottom to 1,194 feet at 

 Brandon on the Assiniboine River. It is significant that the present 

 fall of the Missouri River in an equal distance is much less. The 

 fact that Medicine Lake is now 1 2 feet higher than the flood plain 

 at Poplar is probably due to postglacial erosion by the Missouri 

 River. Another factor which must be considered is that the alti- 

 tudes given do not indicate definitely the position of bedrock 

 because of the varying thickness of glacial drift and alluvium in 

 the old channel. It is also possible that differential warping to the 

 amount of 200 feet, known to have occurred between latitudes 47° 

 and 51° N., in the area occupied by Lake Agassiz and Lake Souris 

 affected this region also and has relatively raised the country to 

 the north from what it was in late Tertiary time.^ 



Changes in YeUowstone River due to glaciation are apparently 

 few. Its present course, to within a few miles of its mouth, is about 

 the same as it was in preglacial time. However, its anastomosing 

 channel from Glendive to Mondak, on a flat, sandy, valley floor, is 

 evidence of glacial filHng in this portion of its channel. From a 

 few miles south of Mondak the Tertiary Yellowstone probably fol- 



' Warren Upham, "The Glacial Lake Agassiz," U.S. Geol. Survey, Mon. 25, pp. 

 474-522, 1896. 



