i 



CRETACEOUS SYSTEM 455 



On the northeast slope of the Shirley uplift the upper sandstone of the 

 Benton is 30 feet thick, and it crosses Muddy creek as a prominent ledge. 

 It is separated from the Mobrara, as usual, by 160 to 200 feet of dark 

 sliale with concretions. Along the southern and eastern flanks of the 

 Freezeout hills the Mowry beds lie 600 feet below the upper sandstone. 

 In this region and again west of Difficiilty there is a local bed of sand- 

 stone in the lower part of the Benton shales. North of the mouth of 

 ]\Iedicine Bow creek the Mowry beds are about 60 feet thick and lie on 

 200 to 300 feet of dark shale with thin local sandstone bodies. They are 

 overlain by 800 feet of shale, followed by the upper sandstone member 

 and aljout 120 feet of dark shale with concretions, which extends to the 

 base of the Mobrara. The upper sandstone member is prominent in the 

 Eock Creek district, giving rise to a long sinuous ridge. ISTortheast of 

 Eock Eiver station it consists of a 4-foot ledge of massive, buff sandstone, 

 and the overl3dng shale is 350 feet thick. The underlying shale contains 

 concretions and a persistent- bed of bentonite M^hich is 4 feet thick for 

 some distance. 



Laramie basin. — The Benton shale occupies a large portion of Laramie 

 basin and it outcrops at intervals from the foot of Jelm mountain past 

 Laramie and Howell. It outcrops along both sides of Centennial valley 

 and appears in small areas in the valley near Jelm and on the slopes west 

 of Eed mountain. There are extensive exposures in tlio vicinity of Hut- 

 ton lake, southwest of Laramie; in the hills I14 miles southwest of 

 Howell, and in the railroad cuts a mile north of Howell. In the south- 

 ern part of the basin the thickness is 700 feet, judging by a boring 10 

 miles southwest of Laramie, Avhich began near the top of the formation 

 and reached the Cleverly sandstone at a depth of 600 feet. The thick- 

 ness of the steeply dipping beds in Centennial valley is between 500 and 

 600 feet. The rocks are mostly gray to l)lack shales. The Mowry beds 

 are about 200 feet above the base and consist of al)out 100 feet of the 

 usual hard shales and thin-bedded, fine-grained sandstones, which weather 

 to a light, silvery gray color and contain large numbers of fish scales. 

 The ridges due to this member are especially conspicuous east of Jelm 

 mountain, east and northeast of Hutton lakes, southwest of Howell, and 

 in the anticlines near Medicine Bow, old Eock Creek station, and Freeze- 

 out hills. The sandstone member near the top of the formation is 20 to 

 30 feet thick and there are usually one or more thin beds of sandstone in 

 tlie lower shales. Southwest of Howell station there are at the base 200 

 feet of dark shales, including a 5-foot bed of buff sandstone 45 feet above 

 the top of the Cleverly sandstone. Next above are 100 feet of Mowry 

 beds overlain by 100 feet of dark shales with dark concretions. Higher 



