CENOZOIC HISTORY OF WIND RIVER MOUNTAINS 143 
shales which comprise the lower part of the Chugwater. In the 
Cretaceous certain sandstones make low ridges, but not at all com- 
parable with those made by the Chugwater. When the Paleozoics 
are concealed by the Tertiary overlap the topography is simpler, 
and the rise from plains to mountains more abrupt. 
The observations which are recorded in this paper were made in 
August and September, 1911, on a trip along the base of the range 
from the headwaters of the Sweetwater River on the west to Bull 
Lake Creek on the north, with side trips into the range and out onto 
the plains. The time which could be given to the work was far 
from sufficient to determine the history of the range fully, and many 
questions which came up had to be left unanswered. It is thought, 
however, that the general history of the range, as read in its topog- 
raphy and that of the surrounding plains, has been made out, and 
that the results are worth making public. We are glad to be able 
to express our appreciation of the courtesy of Mr. N. H. Brown of 
Lander, whose maps of Fremont County and of the vicinity of 
Lander were used in our work. The Fremont quadrangle is the only 
topographic sheet published by the U.S. Geological Survey which 
covers any part of the Wind River Mountains. As our field work 
was to the south of that area, and we were without topographic 
maps (except those of the Hayden Survey, which were too general- 
ized for our uses), we were obliged to get altitudes by the use of 
aneroids, and with less opportunity than we would have wished to 
check such observations by bench marks. 
SEDIMENTARY HISTORY: CAMBRIAN TO EARLY TERTIARY 
Before the end of Cambrian time a wide transgression of the 
sea brought conditions of marine sedimentation over the area of 
the present Wind River Mountains, and from then on to the close 
of the Mesozoic there was almost continuous sedimentation. If 
at times deposit was interrupted, as seems probable from the failure 
to find Silurian and Devonian beds in the series, this did not inter- 
fere with the production of an essentially parallel series of sedi- 
mentary rocks. The youngest rocks found in the Lander region 
are the Mancos shalet (Colorado), over 6,000 feet, and the Mesa 
t Woodruff, Bulletin U.S. Geol. Survey, No. 452, p. 10. 
