276 University of Kansas Geological Survey. 
part of the whole area is covered by sand in the form of sand dunes 
that are so prominent along the south side of the Arkansas river and 
elsewhere, so that it will not do to assume that plains marl is a 
continuous formation covering the whole surface. 
It is probable that many of the properties of the plains marl 
are largely due to the action of wind. Strong winds which are 
common in the plains country continually exercise a sorting action 
on the surface materials, effecting a separation of the finer from © 
the coarser. It is probable that many of the sand dunes owe their 
existence as such largely to the finer silt and soil having been 
blown away, leaving the sand behind as a residual property. Corre- 
spondingly the accumulation of the fine material on the surface in 
other parts of the plains may be to a great extent an important 
factor in the production of the plains marl. The winds therefore 
have been an important agent in the arrangement of the surface 
materials, and probably continue that function to the present time. 
Could we have had an absence of wind over the whole of the plains 
from the close of Tertiary time to the present it might well be 
assumed that the division of the surface material into sand dunes 
and plains marl would scarcely have been possible. From these 
considerations it may be concluded that the plains marls in them- 
selves have but little stratigraphic importance, although they are 
so prominent a surface feature over wide areas at the prsent time. 
THE SAND DUNES. 
The location and general character of the sand dunes along the 
southern side of the Arkansas river were fully described in the 
article on physiography while describing that river. Other sandy 
areas are found here and there over the state which are of more 
than passing interest. On the eastern side of Crooked creek, from 
the vicinity of Wilburn to near Fowler, a sand area occurs which 
covers from twenty to thirty square miles, and which in many 
respects is very similar to the Arkansas area. Along the Cimarron 
river, particularly in the western part of its course in the state, 
sandhills abound. This is true in Grant county, and in Stevens, 
Morton, and Stafford counties, but perhaps more particulary true 
in Stevens county than elsewhere. Here on the south side of the 
