STRATIGRAPHY. 35 
just west of Jelm Mountain, on the slopes 3 miles west of the summit 
of Red Mountain, and along the roadside 3 miles northeast of the 
Rock River depot. The outcrop crosses the Union Pacific Railroad 
just north of Wyoming and at two localities a few miles east of 
Medicine Bow. In many places the formation is buried beneath 
later deposits. The most extensive exposures are in the south end 
of Big Basin, in the east and west portions of The Big Hollow, in 
bluffs along Laramie River west and north of Hutton Lakes, north 
and east of Medicine Bow, in the banks of Rock Creek southwest of 
Rock Creek post-office, on Sheep Creek 2 miles above its mouth, and 
in bluffs along Little Medicine Bow Creek in T. 26 N. 
The formation consists largely of impure chalk rock which weathers 
to a bright-yellow color and contains large numbers of Ostrea congesta. 
The thickness could not be ascertained satisfactorily, except in the 
sharp upturn 2 miles northwest of the Union Pacific Soda Lakes, 
where it is 425 feet, and east of Miser station, where it is about 200 
feet. In the middle of the formation there is a deposit of dark-gray 
shales. The chalk rock is in beds varying from thin layers to slabs 
an inch or 2 inches in thickness, and locally it is sufficiently hard to 
give rise to buttes of considerable prominence, as in the west end of 
The Big Hollow; in bluffs overlooking Big Basin, 5 miles northwest 
of Laramie; and at Chalk Bluffs, 6 miles southeast of old Rock 
Creek station. The bright-yellow color of these bluffs renders them 
conspicuous features. 
MONTANA GROUP. 
Distribution and general relations. — The large syncline northwest of 
Laramie is occupied by shales and sandstones of the Montana group. 
Another smaller area extends along the west side of the valley of 
Little Medicine Bow Creek north of the uplift of the Freezeout Hills. 
The group is here composed of two parts. The lower part consists of 
dark shales representing the greater portion if not all of the Pierre 
shale. The upper part begins with a prominent bed of sandstone 
succeeded by sandstones and shale, including beds of coal, and 
approximately represents the Fox Hills. It is probable that the 
uppermost beds in the center of the larger basin and along its western 
margin represent the Laramie formation. 
Outcrops of the Montana group are numerous and extensive, except 
along portions of the valleys of Little Laramie and Laramie rivers 
and Rock Creek and in some of the intervening divides where their 
are extensive mantles of alluvium and earlier Quaternary deposits. 
In the deeper portion of the basin, south of Lookout, the Montana is 
covered by Tertiary deposits. For many miles along the foot of 
Medicine Bow and Sheep mountains the group is faulted against the 
granite and schists. 
