STRATIGRAPHY. 13 
CARBONIFEROUS SYSTEM. 
CASPER FORMATION. 
General character. — The name (Jasper is here proposed for the 
rocks of Carboniferous age (chiefly Pennsylvanian) capping Casper 
Mountain and extending along both sides of the Laramie Range. 
For the greater part of its course the formation consists of light- 
colored limestones and dolomites merging downward into gray and 
brown sandstones which lie upon the pre-Cambrian crystalline 
rocks. In their southern extension, as shown by Knight, /these 
sediments gradually change into "Red Beds," which consist largely 
of coarse red sandstones south and southwest of Laramie. At the 
top, in the vicinity of Laramie and for some distance north and 
south, there is a conspicuous and persistent bed of limestone which 
is quarried for lime. 
The Casper formation occupies the western slope of the Laramie 
Mountains, a prominent range which from Sybille Creek southward 
has an average height of 1,000 feet above the Laramie Plains and 
constitutes the main divide. The south end of the area occupied by 
this formation extends about half a mile south of the Colorado line 
in the narrow syncline between Boulder Ridge and the Laramie 
Mountains. The syncline between Boulder Ridge and the Medicine 
Bow Mountains also is occupied by the Casper formation, the out- 
crop of which is mostly in Colorado. Irregular outcrops occur 
around the base of Jelm, Ring, and Sheep mountains and the forma- 
tion appears along the foot of the mountain slopes on both sides of 
the Centennial Valley. Along the fault at the foot of the Medicine 
Bow Mountains, north of the Centennial Valley, the formation lies 
at great depth below the surface. The upper sandstone appears in 
an outcrop several miles long in the center of the anticline north of 
Medicine Bow, in the ridge known as Flattop. 
Tliickness. — The thickness of the Casper formation varies con- 
siderably. The maximum observed was in Gilmore Canyon south- 
east of Laramie, where l,007i feet were measured. At the head of 
Sybille Creek and north of Laramie River the amount is considerably 
less than this, being apparently not more than 500 feet. On Sand 
Creek, near Pulpit Rock, the thickness is 500 feet, and it is about 600 
feet in a section measured on the northwestern slope of Red Moun- 
tain. An approximate measurement a mile west of BoswellV ranch 
on Laramie River, at the Colorado state line, gave 1,000 feet and 
another in the Centennial Valley about 850 feet. 
"Knight, W. C, The Laramie Plains Red Beds and their age: Jour. Geology, vol. in, L902, pp. 413-422. 
