STRUCTURE. 49 
north end of Jelm Mountain until they reach the fault which cuts 
across the south end of Sheep Mountain. 
West of the anticline of Jelm Mountain there is a narrow syncline 
holding beds from Casper to Niobrara dropped by a pair of faults, 
as shown in section 5, Plate VIII. Jelm post-office is on alluvium 
on Benton shales, with a continuous succession of formations to Chug- 
water exposed at intervals on the adjacent slopes. The faulting is 
plainly shown on the west side within a mile north and south of the 
post-office, where all the formations are cut off diagonally by the 
schists. Two miles farther north the Niobrara occurs, dipping gently 
eastward and faulted against schists on the west side of the valley. 
From a point a mile south of Jelm northward to Woods Landing the 
syncline has been squeezed and faulted in various ways and the 
structure is very complicated. The details are largely concealed by 
the steep slopes of detritus from the hills. To the south the valley 
widens out and the structure becomes more simple. About Jelm 
the syncline is well marked by the outcropping upturned edges of 
the Morrison and Cloverly formations. It is closely compressed with 
vertical dips on the west side and 45° dips on the east side, and the 
width from one outcrop of the Cloverly to the other is less than one- 
half mile. A mile east of Jelm there is another syncline of Cloverly 
very closely compressed with vertical dips on either limb. The axis 
bears north and northwest and is apparently marked by a narrow 
outcrop of Benton. Just east of this syncline the Chugwater forma- 
tion is faulted against vertical sandstone beds of the Cloverly. The 
Morrison does not appear, probably being faulted out of sight. 
Relations indicating two faults are exhibited half a mile south of 
Woods Landing, where on the east side of the valley the Chugwater 
red beds are in contact with the schists, toward which they dip at 
an angle of 45°. Lower down the slope the Niobrara is exposed, 
dipping in the same direction. 
CENTENNIAL VALLEY. 
The Centennial Valley consists of a symmetrical syncline with dips 
of 40° to 50° W. along the flank of Sheep Mountain and of 35° to 80° E. 
at the foot of the Medicine Bow Mountains. The succession of rocks 
is complete from Casper to Montana along the east side of the valley, 
but on the west side faulting at a point 3 miles south of Centennial 
cuts out the Casper formation entirely. The syncline is cut off by 
faults at both ends of the valley. At the southeast end the older 
formations pass beneath the Tertiary so that their contact with the 
crystalline rocks is not seen, but along the southwest edge of the valley 
the fault is plainly indicated, the granite coming down to the floor of 
the valley along a diagonal line so that finally it is in contact with the 
58050— Bull. 3G4— 09 4 
