CENOZOIC HISTORY OF WIND RIVER MOUNTAINS 147 
The remnants of the plain fall into a low arch (Fig. 2), which 
slopes east from over 12,000 feet along the crest to above 10,000 feet 
in its last remnants on the edge of the crystallines and the bordering 
Bighorn limestone, and drops to the northwest along the axis of 
the range. 
The date of the summit peneplain cannot be fixed, from the area 
studied, more closely than that it is later than the main folding 
of the range; for it does not extend beyond the Paleozoic rim to 
any point where later deposits rest upon it. Blackwelder™ has 
described a peneplain in the Laramie region of southeastern Wyo- 
ming, which makes the summit of the Laramie Mountains at 9,000 
feet and cuts the Medicine Bow range at 10,000, with the summits 
of the latter rising above it as monadnocks. This peneplain is 
considered to be of post-Eocene age. Rich? has described a pene- 
plain in southwestern Wyoming, which had been developed 
probably by the end of Miocene, while Baker, in a paper read 
before the Cordilleran section of the Geological Society in 1911, 
describes the occurrence of a peneplain throughout southwestern 
Wyoming in the end of the Miocene. Umpleby* describes an 
old erosion surface in West Central Idaho which may prove to be a 
peneplain and which he considers to be of Eocene age. It is perhaps 
as much as the facts known at present warrant, to state that the 
Wind River Mountains had been reduced to a peneplain, except 
for a few low residuals along the present divide, by mid-Tertiary 
time. 
THE PLAINS AT THE SOUTH END OF THE RANGE 
Plain No. 4.—The oldest plain below the summit peneplain 
is best represented north of Atlantic City, in the summit accord- 
ance of a hilly area above which the southern end of the range 
rises abruptly (Figs. 3, 4, and 5). These summits stand at 8,500 
feet farthest from the range, but rise to probably 9,000 feet at its 
base; and the original plain has been maturely dissected. East 
of Atlantic City, peaks of Paleozoic limestone rise to the level of 
this plain at 8,500 feet; and still further east the summit of Sheep 
t Journal of Geology, XVII, 420. 
2 Tbid., XVIII, 601. 3 [bid., XX, 139. 
