vi 
STEATIGRAPHY. 43 
TERTIARY SYSTEM. 
There are several deposits of Tertiary age in the region to which 
this report relates, but, except in the valley of the Little Medicine, 
their age has not been ascertained. 
COOPER LAKE BASEN. 
Beginning on the ridge north of Fourmile Creek and extending 
northward beyond Dutton Creek there is a basin of about 125 square 
miles, containing a Tertiary deposit probably of Bridger or Wasatch 
age. The rocks are sandy clays, buff, bright blue, yellow, and terra 
cotta in color, with irregular concretionary bodies of sandstone and 
lenses of claystone. They rest upon or are intercalated with beds 
of sand and conglomerate. The clay members give rise to extensive 
badland areas, especially along the south side of Cooper Creek. The 
beds appear to lie nearly level, except along a portion of the south 
side of the area, where they show very low dips to the northwest. 
The formation lies unconformably upon the Montana group, but, 
except along its western margin, there is no great discordance in dip. 
Half a mile northeast of the north end of Sprague lane, in the NE. | 
sec. 35, T. 19 N., R. 75 W., the bluff shows the following section : 
Section of Tertiary beds in the NE. \ sec. 35, T.19 N.,R. 75 W. 
Feet. 
Terrace gravel and soil 2 
Buff sand with pinnacles of indurated sandstone, some coarse 
grained 10 
Irregularly deposited coarse dark terra-cotta nodular sandstone £-1 
Yellow, purple, and terra-cotta sandy clay 20 
Soft conglomerate, thickness unknown, resembling the deposits 
south of Jelm. 
Similar beds outcrop near the east shore of Cooper Lake, about 
2 J miles farther northwest. In the bluff running through the \. \ 
sec. 12, T. 18 N., R. 76 W., along the south side of Cooper Creek 
valley, well-developed badland topography is presented. A small 
butte in this region has the following section: 
Section of Tertiary beds in the N.% sec. 12, T. 18 N, R. 76 II'. 
Feet. 
Blue sandy clay 4 
Yellow and terra-cotta, sandy clay 6 
Sandy blue clay with irregular white sandstone beds 10 
Badlands also appear in the central portion of T. 19 N., R. 76 W. 
Badlands are not developed on Dutton Creek, but near the Empire 
ranch house, in the NE. i sec. 22, T. 19 N., R. 77 W., the fine gravelly 
mortar beds outcrop in banks 10 feet or more in height. The thick- 
ness of the Tertiary in this region is probably between 150 and 200 
feet. No fossils were found in it. 
