2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
formation of early Eocene age on the south by a normal fault of 
large displacement. 
The Wind River formation is a thick clastic one composed of the 
debris eroded from the Owl Creek and Big Horn Mountains during 
at least the later part of their structural deformation and growth. 
The Wind River formation is exceedingly coarse grained in the area 
of its outcrop closest to the mountains and finer grained away from 
the mountains. The strata of younger Eocene age are conspicuously 
different in appearance and composition, consisting of andesitic vol- 
canic material derived from the volcanic centers in the Absaroka 
Range 70 miles to the west-northwest. Although some of the volcanic 
material probably was carried to the northeastern part of the Wind 
River Basin by streams, much of the material may have been trans- 
ported aerially. The rocks include relatively little material eroded 
from the Owl Creek and Big Horn Mountains, which had their pres- 
ent form and very nearly their present topography at the time the 
rocks of middle(?) and late Eocene age were deposited. 
HISTORY OF INVESTIGATION 
The geology of the northeastern part of the Wind River Basin 
(fig. 1) has long been of interest because of the large faunas of 
vertebrate fossils that have been found in the Eocene strata there. 
J. L. Wortman, collecting in 1880 for E. D. Cope (Osborn, 1929, 
p. 160), appears to have been the first collector to enter the north- 
eastern part of the basin. During the following years, classic col- 
lections were made from the Wind River formation in exposures 
along Cottonwood Creek and in a broad area east of Lost Cabin, and 
along Alkali Creek, just south of the map area shown in figure 2. 
These collections later provided the basis for the faunal definition of 
the latter part of early Eocene time. No fossils of younger Eocene 
age were found in early investigations although the younger strata 
are markedly different in lithology from the Wind River formation. 
Granger saw some of the strata now known to be of late * and prob- 
ably middle(?) Eocene age and considered them to be a lower part 
of the Wind River sequence that is exposed on Cottonwood Creek 
(Sinclair and Granger, 1911, p. 105). The large fault that separates 
these two units was not recognized. 
Granger’s mention of the “dull-colored, deeply disintegrated clays 
3 As used by the U. S. Geological Survey, late Eocene is equivalent to the 
Uintan age of most vertebrate paleontologists. The next younger age, Duches- 
nean, is classified by the Survey as Eocene or Oligocene. 
