Geological Observations.— Reagan, 235 
The Arikaree Formation overlies and rests unconform- 
ably on the Oligocene series. It covers the entire southern 
half of the area mapped, save where it is removed by 
erosion at the head of Oak creek. Farther to the north sev- 
eral detached patches also occur. It probably once covered 
the whole region, but must have been very thin along the 
northern border, as the exposures show that it gradually 
thinned toward the north. At Valentine, Nebraska, it is 
over 400 feet in thickness, while at the artesian well it is 
less than 160 feet. It is composed mostly of sand in various 
stages of hardness, ranging from the very hardest, unmeta- 
morphosed rock in the Rattle Snake buttes to wholly un- 
lithified sand in many other places. The latter, however, is 
packed so hard by pressure that a pick can hardly be driven 
into it. At all places where the whole series is exposed, 
it is found to be capped with a very hard calcium cemented 
sand rock from eight to twenty feet in thickness, except in 
the Rattlesnake Butte region where it is quartzyte sand- 
stone. On the whole it seems to be dry delta, dune, and 
river-channel formation instead of lake deposit, as was for- 
merly believed. The whole region seems to have reached 
the ponded stage at the close of the epoch. The strata 
thicken and thin alternately. They are often fan shaped. 
Many that are very thick pinch out in a short distance. 
Many show water sorting; some only for a little ways, oth- 
ers throughout the entire exposure. Others are composed 
wholly of fine dune sand; while others are heterogeneously 
mixed. Many show cross-bedding. In some places the 
strata pitch at a high angle one way for a little distance and 
at another angle a little farther on, notwithstanding that 
the Oligocene immediately underlying them is horizontal. 
In many other cases the strata dip in all directions from a 
common center, the formation indicating that it is likely an 
alluvial fan deposit. 
So far as the writer could determine, there is no evi- 
dence that the formation is lake deposit. On the contrary, 
land snails, bones of the horse, camel, mastodon, and other 
land animals, together with the dune material, seem to indi- 
cate that it is stream, pond and seolian in origin. 
From the observations of the writer, which were not 
