HaAwortu. | Physiography of Western Kansas. 19 
have been removed and carried downward by some process, so that 
ultimately they were carried away by the waters down stream 
where the currents are of some consequence. The vertical walls 
of the arroyos already alluded to seem to stand as indicators of the 
process nature has employed. May it not be that the principal part 
of erosion in the uppermost part of the arroyos has been accom- 
plished largely by a slow creeping of the underground clays and 
soils immediately under the sod covered surface of the arroyo it- 
self? During the rainy season the subsurface materials are satu- 
rated to a higher degree than on the main uplands. The inclina- 
tion of the surface in such places is relatively great, frequently as 
much as thirty or forty feet to the mile. The sod of the buffalo 
grass on the surface is not broken through by what little water 
may flow during the rainy season, and consequently it will hold 
itself together and will not pass downward with any appreciable 
velocity. but immediately under the roots of the grass where the 
soil and the clay are kept moist during a considerable part of the 
year conditions are favorable for a rapid creeping of the materials 
in this particular part of the arroyo. Such a creeping would allow 
the mantle of sod gradually to settle downward, a settling extend- 
ing itself finally to the outermost limits of the arroyo. The re- 
sult would be that this blanket of sod would have an almost ver- 
tical movement and the steep and precipitous walls of the arroyo, 
frequently measuring from 3 to 6 or more feet in hight, would be a 
natural resuit. 
If this explanation is correct, it is readily understood why the 
various arroyos should uniformly have so great a width at their 
source. The creeping process could be extended over the width of 
a hundred feet as readily as over five, and therefore the abruptness 
of the walls which is noted on the sides would also be produced at 
the immediate source, exactly as is shown in the plates already re- 
ferred to. 
Another feature of the Tertiary plains in the west which has 
attracted considerable attention is the frequent sinkholes, or swales, 
or lagoons, sometimes filled with water. The various topographic 
sheets issued by the United States Geological Survey have the 
greater number of these pools marked in blue lines, implying that 
