JURASSIC. 569 



Section of Sundance formation in Freezeout Hills, Wyo. 



Feet. 



15. Purplish clay with sandy inclusions (has sandy hmestone layer near base filled with fossils) . 40 



14. Greenish sandstone, thinly laminated 2-5 



13. Purplish fossiUferous clay, with many lime nodules filled with fossils 20 



12. White, sandy clay, dinosaur [marine saurian] -remains 4 



11. Sandy clay with brown concretions; green sandstone layer near middle 6 



10. Wliite sandy limestone in thin layers (fossils) J 



9. Sandy red clay 10 



8. White fissile sandstone 6 



7. Shale, reddish, changing to purple 4 



6. White sandstone, moderately hard i 



2-5. White sandstone separated by two layers of red clay 1 



1. Red clay of Chugwater formation. 



94+ 



East of Laramie Mountains. — The thin margin of the Sundance formation outcrops con- 

 tinuously along the eastern foothills from North Fork of Horse Creek to the southernmost 

 prong of Horse Creek and from the North to the South Fork of Lodgepole Creek. Small 

 exposures appear on North and Middle forks of Crow Creek. The rocks are mostly light-buff 

 sandstones, but some sandy shales appear. The average thickness is 40 feet. * * * 



Fossils and equivalency. — The formation contains abundant fossils of Middle to Upper 

 Jurassic age, and it is believed to be equivalent to the Sundance formation of the Black Hills 

 region. The following are the principal fossils: Belemnites densus, GrypJixa calceola var. 

 nebrascensis, Campectonites [Camptonectesf] bellistriatus, Eumicrotis curta, Trigonia elegantissima, 

 T. americana, T. conradi, T. montanensis, and Pentacrinus asteriscus. 



In Freezeout Hills the following fossils are reported by W. N. Logan in section given [above] : 

 Bed No. 10, Pseudomonotis curta; bed No. 12, remains of Ichthyosaurus and Plesiosaurus ; bed 

 No. 13, abundant Belemnites densus throughout, and in the concretions Pinna Jcingi, Pinna sp. 

 Cardioceras cordiforme, Avicula heedei, Astarte paclcardi, Pentacrinus astericus, Tancredia iulbosa, 

 T. magna, Lima lata, Goniomya montanaensis, Avicula macronatus, Pleuromya suhcompressa, 

 Oardinia wyomingensis, Pseudomonotis curta, Belemnites densus, and B. curtus, and also remains of 

 plesiosaurs and ichthyosaurs ; bed No. 14, Camptonectes lellistriatus, C. extenuatus, Ostrea densa, 

 and 0. strigilecula; in a sandy limestone bed in base of bed No. 15, Pentacrinus astericus, Asterias 

 dubium, Pseudomonotis curta, Avicula macronatus, and Ostrea strigilecula.'^ 



The Jurassic of the Black Hills is represented by the Sundance formation 



and probably by the Unkpapa sandstone (local) and the Morrison formation, 



although the latter two may prove to be of Cretaceous age. (See Chapter XIV, 

 pp. 606-608.) Darton'"<^ says: 



This name [Sundance formation] has been given to the marine Jurassic sediments of the 

 Black HUls region, from the town in the northern portion of the uplift. The rocks are shales 

 and sandstones in a series, some members of which vary locally. Over a wide area the succes- 

 sion consists of a lower member of dark-gray shales, averaging 50 feet thick; a prominent ledge 

 of fine-graiaed sandstones of pale-buff tint; an intermediate member of sandy shales and sand- 

 stones of reddish color, and at the top about 150 feet of dark-green shale, including thin layers 

 of very fossUiferous limestone. * * * Fossils occur also in the sandstone and all are of 

 later Jurassic age. At the base of the formation there is often a massive red or buff sandstone 

 occurring in extended lenses, and frequently attaining a thickness of 25 feet, lying unconform- 

 ably. * * * 



This formation extends along the slope of the Laramie Range, presenting its usual charac- 

 teristics to the northward but thinning rapidly to the southward. In extensive exposures at 

 the east end of Casper Mountain it has a thickness of about 350 feet. At the base overlying the 



o The spellings of fossil names in this list have been made to agree with those given by Logan in the article 

 cited by Darton (Kansas Univ. Quarterly, vol. 9, 1900, pp. 112-113).— B. W. 



