CRETACEOITS SYSTEM 451 



upper Cretaceoiis. Its thickness averages 1,200 feet. East of longitude 

 107° the challcy sediments of the Niobrara formation characterize its 

 upper portion, hut -westward this hecomes shale or shale and sandstone, 

 not clearly separable from the Benton. Accordingly, in the map, plate 

 22, both formations are grouped as the "Colorado formation," while in 

 ]ilate 21 they are shown separate. The outcrops of the Colorado forma- 

 tion appear on both slopes of the Owl Creek uplift; on the east side of 

 the Wind Eiver uplift south from ISTorth fork of Little Wind river ; on 

 the north slope of Eattlesnake mountain; in the uplifts southwest of 

 Casper; south of Big Muddy, Glen Eock, and Douglas; west of Ervay 

 and on Conant creek ; near Alcova ; on the slopes of the uplift of Freeze- 

 out hills; on the south side of the Shirley hills; in the anticline north 

 and east of Medicine Bow and Eock Creek ; along the east side and across 

 the south end of Laramie basin; in the Centennial syncline, near Jclm 

 and at intervals along the east side of Laramie mountains from Chug- 

 water creek to Crow creek. A small outlier remains on the plateau sum- 

 mit of the Haystack range. 



Benton formation. — The Benton formation consists mainly of dark 

 gray fissile shale, but about 200 feet below the top there is a constant 

 horizon of moderately hard, massive to slabby, gray to buff sandstone, 

 which usually outcrops as a distinct "hogback," as shown in plate 30. 

 In the lower beds some sandstone and deposits of hentonite occur. The 

 latter is a white or slightly tinted clay or hydrous silicate of alumina of 

 peculiar porous texture. It presents some evidence of being a decom- 

 posed volcanic ash. There is also, near or Ijelow the middle of the for- 

 mation, a persistent and characteristic memlier, the Mowry beds, which 

 consists of from 100 to 200 feet of hard, gray shales and fine-grained, 

 thin-bedded sandstone, which weather to a light silvery gray. They con- 

 tain large numbers of impressions of fish scales. Owing to their hard- 

 ness, the Mowry beds usually give rise to prominent ridges which bear 

 scattered pine trees. Lens-shaped concretions occur in the loM'er and 

 upper Benton, especially in the shale above the Mowry beds. The Green- 

 horn limestone, Avhich is such a distinct horizon in the Great Plains re- 

 gion, is lacking, excepting along the cast side of the Laramie mountains. 

 The upper part of the Benton, however, ])resents all the characteristics of 

 the Carlile formation, and the lower part is identical with the Graneros 

 shale. This shale usually begins abruptly above the Cloverly sandstone, 

 and there is considerable evidence of local unconformity. Locally the 

 lower Benton beds include thin, rusty layers of irony sandstones inter- 

 calated among the shale. 



Niohrara formation. — In southeastern AVyoming the Benton shale is 

 succeeded by chalk, limy shale, and limestone, 200 feet or more thick, 



Xlill— Bull. Geol. Soc. Am., Vol. 19, 1907 



