EFFECTS OF CERTAIN CHARACTERISTICS OF ROCKS ON 
THEIR EROSION. 
BY J. E. TODD. 
A study of the streams of the western slope of the Missouri 
valley in South Dakota and northern Nebraska presents some strik- 
ing contrasts. These are the more notable because the histories of 
the streams since the Tertiary have been much the same. At the 
opening of the Pleistocene, their courses were uniformly eastward 
from the mountains on the west to the master stream in the James 
River Valley. The Glacial Period resulted in the shifting of the 
latter westward to the present line of the Missouri above Yankton 
and it rapidly cut down to a grade of about one foot per mile. This 
quickened or rejuvenated all the streams considered. 
We have the more reliable data of the two which are most sharply 
contrasted, viz., the Niobrara and the Cheyenne. These extend 
farthest west but the Grand, Moreau and White resemble more the 
Cheyenne than the Niobrara, in their original features. 
The Niobrara has a quite uniform slope and simple structure, the 
Cheyenne has a gentle slope in its lower and middle course, and 
though its upper course is steeper it does not equal the slope which 
the former averages for its whole course. Moreover it has stolen 
the upper courses of its neighbors till it has monopolized the whole 
drainage of the Black Hills. 
The slopes of the streams estimated from a few reliable data are 
shown in the following table : 
Slope. 
Stream. 
Miles. 
Feet Per Mile. Average. 
Niobrara: 
Lusk to Marsland 
...65 
11.6 
• • • • 
Marsland to Valentine 
...140 
11.8 
• • • • 
Valentine to Niobrara 
. . .130 
10.0 
11.2 
Cheyenne : 
N. Branch — Moorcroft to Stonesville Flats 70 
11.4 
• • • • 
N. Branch — Stoneville Flats to Fork.. 
. . .120 
10.8 
• • • • 
S. Branch — Edgemont to Fork 
...125 
10.4 
• • • • 
Main Stream — Fork to mouth 
...85 
7.7 
9.9 
Reasons for a difference of slope of 1.3 feet per mile or nearly 2 
feet if we restrict our view of the latter to its course from angle of 
( 267 ) 
