SEDIMENTARY ROCKS 471 



To show the correlation of these members with subdivisions already 

 known elsewhere in the State, we have prepared a network of sections 

 which ties together the representative areas. The form of this network 

 is indicated on figure 7, and the sections themselves are given in figures 

 9 to 14, inclusive. The bases of correlation are several — in some areas 

 one member, in some another — being most pronounced and readily iden- 

 tified. One member, however, the Mowry, has everywhere a typical char- 

 acter and a typical fish-scale fauna, and this member has been traced in 

 the field without a break from Wyoming to beyond Great Falls, and 

 where last seen here it occupies an horizon which can with little chance 

 of error be located in the sections to the west and north. This gives a 

 definite datum to which all the Cretaceous sections can be referred. 

 With this preliminary explanation, we can pass to the descriptions of 

 the members of the Cretaceous series. 



In the Sweet Grass Hills the Kootenai formation consists of rusty- 

 weathering brownish shales and buff to gray sandstones. Red color is 

 pronounced only in the uppermost members, and even in them is much 

 less here than to the south and east. The heavy sands at the base are 

 associated with coal seams farther south and southwest, but no coal has 

 been found in the Kootenai in the Sweet Grass Hills. One of the lower 

 sands does, however, contain a tarry oil residue, both here and at several 

 points to the north. The source of this maltha is an unsolved problem. 

 Certainly, the Kootenai itself has no petroliferous or organic rich shales, 

 and the Phosphoria is not present here. Possibly, farther north, the 

 Kootenai rests upon petroliferous Devonian beds which were exposed 

 by the Jurassic erosion, and oil from these might find its way into 

 Kootenai sands and, after migration and the distillation of the volatile 

 constituents, leave the tarry residue now found. 



The Kootenai formation is about 450 feet thick — a thickness which it 

 retains southward and eastward nearly to the borders of Montana. 

 Westward, toward its source, it increases rapidly, reaching 3,900 feet in 

 the Rocky Mountains near Crows Nest Pass, about 150 miles northwest 

 of the Sweet Grass Hills. 



UPPER CRETACEOUS 



Colorado group. — The Kootenai formation is continental, trending 

 toward arid conditions at the top; it is succeeded by the marine sand- 

 stones and shales of the Colorado group. These beds, generally soft and 

 easily eroded, occur as a lowland belt about a mile wide, surrounding 

 each butte, limited outward by an encircling rim of Eagle sandstone or 

 a bench of Quaternary gravels. Most of the igneous masses of the 

 XXXIII— Bull. Geol. Soc. Am., Vol. 32, 1020 



