STRATIGRAPHY. 33 
700 feet, to judge by the drill hole 2 miles northwest of Hutton 
Lakes, which began near the top of the formation and reached the 
Cloverly at a depth of 600 feet. The thickness of the steeply dipping 
beds in the Centennial Valley is between 500 and 600 feet. Near 
Rock Creek the formation is about 1,000 feet thick, and the amount 
increases to 1,300 feet northwest of Medicine Bow. 
Character. — The Benton formation consists mainly of gray to 
black shales, and it lacks the middle limestone member (Greenhorn), 
which is so characteristic in the Black Hills region and Colorado. 
About 200 feet above the base is the Mowry shale member, which 
consists of about 100 feet of hard shales and thin-bedded, fine-grained 
sandstones, which weather to a light silvery gray color and contain 
large numbers of fish scales. Owing to its hardness, this member 
gives rise to ridges of considerable prominence. It is especially con- 
spicuous in the slopes east of Jelm Mountain, east and northeast of 
Hutton Lakes, southwest of Howell, and in the anticlines near Medi- 
cine Bow, old Rock Creek station, and the Freezeout Hills. Near 
the top of the Benton formation there is invariably a bed of sand- 
stone in many places 20 to 30 feet thick, and toward the bottom 
there are usually one or more thin beds of sandstone, one of which 
is several feet thick at Hutton Lake. In the exposure southwest 
of Howell station the following beds were exhibited: At the base, 
200 feet of dark shales, including a 5-foot bed of buff sandstone 45 feet 
above the top of the Cloverly formation; next above, 100 feet of 
Mowry shale overlain by 100 feet of dark shales with dark concre- 
tions. The upper beds are concealed by Quaternary deposits in this 
vicinity, but the upper shales are extensively exposed in the long, 
deep railroad cut a mile north of Howell, and the upper sandstone 
outcrops in cuts just south of Wyoming station, where it is 30 
feet thick. In the slopes 5 miles east of the summit of Jelm Moun- 
tain the basal beds below the Mowry member consist of 20 feet of 
drab, brittle sandy shale, 6 feet of black sandstone, 4 feet of white 
sandstone, and 145 feet of black shale lying on the Cloverly formation. 
In a bluff on the south side of Laramie River, 3 miles north-northeast 
of the summit of Jelm Mountain, the upper beds of the Benton exposed 
consist of 5 feet of black shale at the top, 27 feet of soft gray heavy 
flaggy sandstone, 8 feet of interbedded sandstone and shale, and 30 
feet of black shale. In the syncline northeast of Red Mountain the 
lower 110 feet of the formation consists of black shales, which are 
overlain by 30 feet of yellowish to gray shales and 10 feet of flaggy 
buff sandstone containing impressions of long, narrow willow-like 
leaves. On the southeast side of Hutton Lake the lower beds of the 
Benton are exposed, dipping steeply and lying upon the Cloverly for- 
mation. At the base are 100 feet of dark shale with a few thin sand- 
stone layers, then 25 feet of gray sandstone, overlain by 30 feet of 
58050— Bull. 364—09 3 
