4 BULLETIN 1113, U. S. DEPARTMENT OF AGRICULTUEE. 



glaciated area is for the most part an undulating prairie, except for 

 a belt of rough morainic hills called the " Coteau du Missouri.'' This 

 belt of hills, which marks the limit of the Wisconsin ice sheet, ex- 

 tends diagonally across the State of North Dakota, conforming in 

 general to the course of the Missouri River, but at a distance of some 

 50 miles more or less to the north and east. It varies from 10 to 

 20 miles in width and rises in massive hills and ridges strewn with 

 granite bowlders to heights of 150 to 200 feet above the surrounding 

 prairie. Numerous hollows and undrained depressions between these 

 hills and ridges are occupied by swamps and alkaline and fresh-water 

 lakes. 



Practically all the glaciated area in Montana is rolling prairie or 

 level stretches of bench land, all of which is well drained. The 

 glaciated area is indicated in Figure 1. 



NONGLACIATED SECTION. 



The remainder of the region lying between the Missouri River and 

 the foothills of the Rocky Mountains is untouched by the action of 

 the glaciers, and the surface features are almost wholly the result of 

 erosion. It is an area of rolling and for the most part treeless 

 prairies or broad sweeping valleys. The entire section is drained 

 by the Yellowstone and Missouri River systems. So complete a 

 system of watercourses is formed by the tributaries of these rivers 

 that practically the entire surface takes the form of slopes leading 

 into some drainage basin. 



Several isolated groups of small mountains rise abruptly from the 

 prairie at a considerable distance from the frontal range of the 

 Rocky Mountains. They are the Black Hills in South Dakota ; the 

 Highwood Mountains, the Bearpaw Mountains, the Sweet Grass 

 Hills, and the Larb Hills in Montana. These groups are described 

 by the United States Geological Survey as, for the most part, igneous 

 intrusions and not connected directly with the Rocky Mountains. 

 With the exception of the Larb Hills, they are all covered with a more 

 or less heavy stand of coniferous trees. The heavy growth of west- 

 ern yellow pine that once covered the Bearpaw Mountains now shows 

 the marked effect of thinning through the combined work of settlers 

 and fire. 



Another feature of this nonglaciated section is the large number of 

 conical or flat-topped buttes rising several hundred feet above the 

 general level and forming prominent landmarks that can be seen for 

 great distances. These buttes were formed by the erosion of the 

 surrounding soil and are remnants of the ancient Missouri Plateau 

 that once covered the entire section. Notable examples of this for- 

 mation are the Killdeer and Turtle Mountains in North Dakota, the 



