CRETACEOUS SYSTEM 459 



Wind River basin. — The broad syncline between the uplifts of Wind 

 Eiver and Owl Creek mountains holds a great thickness of Montana for- 

 mation, but it is mostly covered by Tertiary deposits. Outcrops appear 

 in the valleys of Muddy and Dry creeks, in the broad belt extending 

 through Fort Washakie and Lander, and on the uplifts on Conant and 

 Muskrat creeks. The lower division, or Pierre shale, consists of a monot- 

 onous succession of soft, dark gray shales with intercalated thin beds of 

 sandstone in their upper part. Their thickness is 2,600 feet on Dry 

 creek, 2,250 feet on Muddy creek, where the beds are nearly vertical, and 

 over 3,000 feet east of Lander. The upper division consists of sand- 

 stones with intercalated gray shales and sandy shales and local coal beds. 

 The first bed of sandstone, from 200 to 250 feet thick, is usually suc- 

 ceeded by sandy, carbonaceous shale which develops locally into coal. 

 This coal is 8 feet thick for a short distance in the coal field 10 miles east 

 of Lander and about 3 feet thick on Muddy creek. The upper division 

 attains a thickness of 550 feet before it disappears under the Tertiary 

 east of Lander, and about the same amount appears in the ridges on 

 Muddy creek. In the uplifts on Conant and Muskrat creeks and on the 

 north side of Eattlesnake mountains the Montana formation presents feat- 

 ures similar to those described above. 



Laramie basin. — In the deep syncline west and northwest of Laramie 

 the lower division of the formation consists of 2,500 to 4,000 feet of dark 

 shales, with occasional thin beds of sandstone and numerous nodular con- 

 cretions. The sandstones are seldom over 5 feet thick and are separated 

 by from 100 to 300 feet or more of dark shales. Toward the top the 

 sandstones rapidly increase in number and frequency, and in places they 

 constitute a passage series into the upper division. The latter usually 

 begins with a prominent bed of hard, gray sandstone 60 to 80 feet thick. 

 This sandstone outcrops most conspicuously in "Pine ridge," which ex- 

 tends along the Union Pacific railroad 2 to 3 miles southeast of Eock 

 Creek station; on the bank of Little Laramie river 17 miles northwest of 

 Laramie ; in a high knob 4 miles northeast of Sheep mountain ; in a ridge 

 extending southward from Eock Creek station, and in the syncline along 

 the foot of Medicine Bow mountains 10 miles northeast of Centennial. 

 It crosses Laramie river at Dun's ranch, 13 miles southeast of Eock Creek 

 station. Generally this sandstone contains concretions of harder rock 

 and some darker gray layers. It is overlain by a variable thickness of 

 softer sandstone of various kinds, followed by a succession of shales and 

 sandstones. The amount of sandstone in this member increases to the 

 southward. Some of the rocks are carbonaceous and locally develop into 

 coal from a few inches to 4 feet or more thick. The most persistent coal 



