UPPEK CRETACEOUS. 689 



and varying in age from lower Montana to Tertiary. These in turn are overlain 

 by the Fort Union. 



The Kootenai (Lower Cretaceous) and Colorado formations extend northwest- 

 ward from the fields described and have been traced around the Big Belt Mountains, 

 according to Fisher/^^'' who says (see also Chapter XIV, pp. 621-623) : 



The Colorado shale rests with apparent conformity upon the underlying Kootenai and is 

 overlain conformably by the Eagle sandstone, the lowest member of the Montana group. 

 Although conformable relations appear to exist between the Kootenai and Colorado formations 

 in this region, the Dakota, which occupies a position between these two formations in other locali- 

 ties, is, as previously stated, believed not to be present. If this is true, there is a hiatus at this 

 contact representing at least several hundred feet of beds. It is possible that Dakota time is 

 here represented by marine sediments not easily separable from the Colorado shale. 



The Upper Cretaceous section of central Montana was established by Stanton 

 and Hatcher ,^^® from whose report the following general facts are abstracted: 



The Benton (or more properly the Colorado, as more than the Benton is 

 included) consists of dark clay shales with intercalated bands of sandstone, espe- 

 cially in the upper portion, and frequent calcareous concretions. The fossils of the 

 lower portion are Inoceramus lahiatus, Scaphites warreni, and others. The upper 

 part yields Inoceramus umbonatus, I. exogyroides, Pholadonya papyracea, Scaphites 

 ventricosus, Baculites asper (?), and many other forms. The total thickness is 

 estimated at 800 feet. 



Immediately above the Colorado is the Eagle formation. At its base the Eagle 

 consists of sandstones and shale which grade into the underlying shale, but its 

 conspicuous member is a massive light-colored cross-bedded sandstone, more than 

 100 feet thick. Above this member are softer beds of sandstone together with shale 

 and lignite. Fossils are scarce but include, of invertebrates, Cardium speciosum, 

 Thetis 'icircularis, Mactra alta, Placenticeras whitfieldil, and others, and, of verte- 

 brates, remains of the dinosaur Ornithomimus grandis. The thickness is probably 

 300 feet or more. 



The Claggett formation is a body of dark clay shales, about 400 feet thick, 

 which rests on the Eagle. In the upper part are yellow sandstone beds, and the 

 passage into the Judith River is by gradual transition. The lower part of the 

 Claggett contains Gervillia borealis, Baculites ovatus, Baculites compressus, and other 

 characteristic Pierre forms: The sandstone beds contain a fauna which " has long 

 been considered a typical 'Fox Hills' fauna." Stanton concludes that "faunas 

 similar to that of the Fox Hills sandstone have a great vertical range and are likely 

 to be found at any horizon within the Montana group where a littoral or shallow- 

 water facies is developed." 



The Judith River consists chiefly of fresh-water deposits together with layers 

 laid down in brackish water, especially near the top. There are also local marine 

 beds. Workable coals occur in the upper portion. The thickness is about 500 feet. 

 Extended descriptions of the Judith River formation are given in the bulletin cited. 



The Bearpaw shales are marine and succeed the prevailingly fresh-water 

 Judith River. They are dark clays which include many calcareous concretions. 

 The thickness is estimated at 750 feet. The commoner forms in the varied inverte- 

 brate fauna are listed in Stanton and Hatcher's report. 



48011°— 12 44 



