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