GEADATIOSrS FROM CONTINEIfTAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 



15 



about halfway between the western margin of 

 the area and Musselshell River and is not ex- 

 posed farther east, but at the Musselshell, 

 where there is a clean-cut exposure from the 

 Lance down mto the Colorado, the Eagle is 

 either lacking or is represented by a sandstone 

 3 feet thick. 



The pinching out of the Eagle is not due to 

 an unconformity but to the seaward thinning 

 of a shoal deposit laid down during a reces- 

 sional stage of the Cretaceous sea and later 

 covered by an off-shore deposit as the sea 

 deepened and the shore line was shifted farther 



west. 



CLAGGETT FORMATION. 



The Claggett consists of 600 to 700 feet of 

 dark-gray shale containing numerous calcareous 

 concretions hi which invertebrate fossils occur. 

 The shale is but slightly consolidated and 

 becomes very plastic when wet, so that it is 

 commonly referred to as "gumbo." Crystals 

 of selenite are more or less abtmdant in the 

 shale and hi places accumulate in considerable 

 quantities on weathered slopes. At the top of 

 the formation there is a zone that is especially 

 prolific in large calcareous concretions, many 

 of which show a well-developed cone-in-cone 

 structm"e. This feature, so far as the writer is 

 aware, is peculiar to the Claggett throughout a 

 large part of Montana and does not occur m 

 either the Colorado or the Bearpaw. It there- 

 fore constitutes one criterion for the recogni- 

 tion of the Claggett in central Montana. 



JUDITH RIVER FORMATION. 



Character and thickness. — The Judith River 

 formation, as originally defined,^ includes fresh 

 and brackish water beds lying above the marine 

 Claggett and below the Bearpaw. This defini- 

 tion was given before the full extent of the 

 formation was known. The work of the season 

 of 1918 has shown that in the area described 

 the fresh-water deposits when traced eastward 

 grade into marine deposits of the same litho- 

 logic character. In this paper the term Judith 

 River is used to include both the fresh-water 

 deposits exposed in the western part of the 

 area and their equivalent marine deposits in 

 the eastern part. 



The fresh-water facies may be considered as 

 m.ade up of three parts — a lower division of 



1 Stanton, T. W., and Hatcher, J. B., Geology and paleontology of 

 the Judith River beds: U. S. Geol. Survey Bull. 257, pp. 33-34, 1905. 



massive brownish, poorly cemented sandstone, 

 a middle division of ash-gray shale, and an 

 upper division of grayish-white to brown mas- 

 sive to heavy-bedded sandstone. The total 

 thickness of this facies of the formation is 

 about 185 feet. 



The marine f*acies likewise has a threefold 

 character which corresponds in every way to 

 the fresh-water divisions except that the middle 

 shale member is darker. The upper sandstone 

 is of a light-brown color in the eastern part of 

 the field but farther west changes to light gray 

 or dirty white; the lower sandstone is prevail- 

 ingly brown on weathered surfaces. The 

 maruie facies has a thickness of about 125 feet 

 near the head of Big Porcupine Creek; it is 

 reported as 200 feet thick in the Vananda weU 

 and may be thicker than that, as it is believed 

 that the mouth of the well is below the top of 

 the formation; but on the east side of the field 

 the thickness probably does not exceed 100 

 feet. 



Detailed sections of these two facies are 

 given below. 



Section of Judith River formatio7i (marine) in sec. 8, T. 12 N. , 

 R. 38 E. 



Feet. 

 Sandstone, massive, yellow to brown (marine shells 



and Ealyvxenites) 20 



Talus, sandy, probably sandstone 30 



Shale, gray to yellow 20 



Sandstone, massive, gray at top and full of Halyme- 

 nites, lower part not well exposed 45 



115 



Section of Judith River formation (fresh-tvater) on west bank 

 of Musselshell River, in sec. 16, T. 15 N., R. 30 E. 



Shale, Bearpaw. Feet. 



Sandstone 29 



Shale, light gray 15 



Shale, carbonaceous SJ 



Sandstone 8 



Shale, gray; carbonaceous in upper part 13 



Sandstone 9 



Shale 35. 



Sandstone 13 



Shale, light gray 15 



Sandstone 2 



Shale 3 



Shale, carbonaceous 8 



Sandstone 31 



Shale, Claggett, dark gray, sandy at top, transition. 



189i 



Composition. — As revealed by a study of 

 thin sections the sandstones of both the fresh- 

 water and marine facies of the formation are 

 as similar in microscopic appearance as they 



