16 University of Kansas Geological Survey. 
different parts of the state. Throughout the western part over the 
areas covered with the Tertiary erosion could be produced very 
rapidly were the precipitation sufficient. Much more than half of 
the Tertiary material is in the form of loose beds of silt, or clays, or 
sand, or gravel, materials which would yield most readily to the 
corrasive action of running water. 
Elsewhere the Cretaceous rocks are exposed to the surface, or 
have been so exposed. These in general would resist corrasion more 
vigorously than the Tertiary materials. In places, it is true, 
they are composed principally of shales which would yield quite 
readily. But elsewhere the limestone beds are so frequently inter- 
spersed with the shale that a relatively strong resistance to all forms © 
of erosion would occur. It is quite probable that such formations 
have had a strong influence in producing the particular physiog- 
raphy of the country wherever they are exposed to the surface. 
Eastward from the Benton and Niobrara exposures we find the 
Dakota covering the surface. In most places the materials of the 
Dakota are quite easily worn away. Consisting as they do of sand- 
stone and shales, with the sandstone weakly cemented together, 
they offer but little resistance to the vigorous action of running 
water. Ina few horizons the cementing material of the sands seems 
to be sufficiently strong to produce a tolerably strong sandstone, 
and therefore a material which will resist decay. It is a noticeable 
fact that the character of the river channels vary from place to 
place through their course, depending to a great extent upon the 
- character of the material through which the channel is worn. 
The residual materials found here and there through the drainage 
area corresponds in character quite closely with the materials into 
which the channe! has been produced. Through the Tertiary area 
the most common residual products are sand and gravel. It is by no 
means unusual to find a small stream, or an arroyo, of only a few 
miles in length, in the Tertiary area which has carried away the 
clay and finer silt and has left behind as a complete mantle for its 
flood plain the sand which constitutes a portion of the original 
materials. Such sand washes are seen particularly in the southern 
part of the state through Meade and Clark counties, where the 
inclination of the surface to the south is so great that the streams 
