12 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 



stone, which does not extend east of Mussel- 

 shell River, these formations are believed to be 

 continuous throughout the area, but the Judith 

 River changes from a fresh-water formation in 

 the western i)art of the area to a marine forma- 

 tion in the eastern part. In other words, 

 marine conditions prevailed over the eastern 



in the immediate vicinity, but have been pene- 

 trated by the drill to a depth of about 2,500 

 feet below the top of the Colorado shale in a 

 deep well put down at Vananda, in T. 7 X., 

 R. 38 E., by the Chicago, Milwaukee & St. 

 Paul Railway Co. The greater part of this 

 thickness consists of the dark-gray to black 









Geologic format 



ons in the area 



herein discussed. 



System and series. 



Group. 



Formation. 



Thickness 



(feet). 



Character. 



Quaternary. 









Alluvial gravel, sand, and silt alonsr Yellowstone 

 and Musselshell rivers and some of the smaller 

 streams. 



Tertiary (?) (Eo- 

 cene?). 





Lance formation. 





Brown, irregularly bedded sandstone, alternating 

 with "somber" gray shale. Upper part (25 

 feet or more) is somewhat sandy, forming a 

 transition to the Lance. This transition phase 

 is prol)ably what has been recognized as Fox 

 Hills on Hell Creek about 35 or 40 miles farther 

 north. 





Upper Creta- 

 ceous. 





Bearpaw shale. 



900-1, 100 ± 



Dark-gray shale in which occur calcareous concre- 

 tions containing marine invertebrate fossils. 





03 



d 



o 



Judith River for- 

 mation. 



100-200 ± 



Upper member, light-brown to light-gray massive 

 sandstone. Middle member, light-gray to dark- 

 gray shale. Lower member, sandstone, weather- 

 ing brown and giving rise to large boulder-like 

 masses. The formation is of fresh-water origin 

 in the western part of the tield and of marine 

 origin in the eastern part. 



3 

 O 



Claggett formation. 



600-700 ± 



Dark-gray shale; contains numerous large cal- 

 careous concretions in upper ])art, some of 

 which show well-developed cone-in-cone struc- 

 ture; marine fossils similar to those in the 

 Bearpaw. 



O 



Eagle sandstone. 



0-21 



Buff massive sandstone; does not occur east of 

 Musselshell River. 







Colorado shale. 



2,300 ± 



Dark-gray to black fissile shale, with thin beds 

 of sandstone and sandy shale in upper part and 

 thin beds of limestone and sandstone near the 

 base; shale is only slightly plastic when wet. 

 Contains marine fossils. 





Lower Creta- 

 ceous (?). 





Kootenai forma- 

 tion (?). 



157+ 



Consists predominantly of red and white shale, 

 overlain by a thin bed of limestone and sand- 

 stone. 



part throughout the Montana epoch, whereas 

 in the western part a temporary recession of 

 the sea allowed the accumulation of fresh- 

 water deposits. These conditions are illus- 

 trated in the diagrammatic and generalized 

 sections shown on Plate IV. 



Rocks below the upper part of the Colorado 

 shale are not exposed in this field or anywhere 



shale of the Colorado, but the lower 157 feet 

 seems to represent some underlying formation, 

 probably the Kootenai. 



The accompanying table presents in con- 

 densed form the sequence, character, and thick- 

 ness of the exposed formations and of those 

 penetrated by the drill in the w^cll at Vananda, 

 a detailed log of which is given on page 13. 



