47G KEMP AND EILLINGSLEY SWEET GRASS HILLS, MONTANA 



1. Frontier member — sandy phase near base, best developed in the Bighorn 



Basin. 



2. "A" beds — limy concretions at horizon about 200 feet above Frontier. Trace- 



able from Black Hills to Great Falls. 



3. Greenhorn limestone — thin, platy, fossiliferous limestone, traceable from 



Black Hills to Billings, about 300 feet above "A" l»eds. 



4. Niobrara limestone — at top of Upper Colorado. Not distinguishable in 



Montana, but probably represented by uppermost 100 feet of calcareous 

 shales in the Svi^eet Grass Hills section. 



Montana group. — From the maximum marineward oscillation repre- 

 sented by the Upper Colorado, the pendulum of earth-movement swung 

 back, with several breaks, to the maximum landward oscillation, repre- 

 sented by the Tertiary deposits. The Montana group of alternating 

 terrestrial sandstones and marine shales is the expression of these 

 movements. 



The present knowledge of this group is based almost entirely on the 

 painstaking work of a group of geologists of the United States Geological 

 Survey. The earlier investigators were almost unavoidably confused by 

 the repetition of similar formations, by' the numerous obscure faults and 

 the long stretches of Tertiary deposits, which concealed the Montana 

 group. Thus G. M. Dawson, Hayclen, Peele, and Weed were led to 

 correlate formations similar in appearance, but as widely separated in 

 time as Colorado, Claggett and Bearpaw or Eagle, Judith River, and Fox 

 Hills. The true sequence was first made plain by Hatcher and Stanton,^^ 

 and the stratigraphy of the group has been elaborated by Stebinger,-" 

 Bowen,-^ and Hancock-^. Calvert and Stone^-'* have attacked and solved 

 the problem of the andesitic deposits of central Montana. The Montana 

 group is one of the best known and most adequately interpreted in the 

 region. 



The Eagle sandstone is the lowest member of the Montana group. It 

 is a light-colored, soft sandstone, about 100 feet thick, and is in the field 

 the best horizon marker of all the upper Cretaceous members. It im- 

 mediately underlies the surface for many miles to the west, north, and 



1" T. W. Stanton and J. B. Hatcher : Geology and paleontology of the Judith River 

 beds. U. S. Geol. Survey Bull. 257, 1905. 



™ Eugene Stebinger : The Montana group of northwestern Montana. IT. S. Geol. Sur- 

 vey Prof. Paper 90, October, 1914, pp. 61-68. 



^ C. F. Bowen : The stratigraphy of the Montana group, with special reference to the 

 position and age of the Judith River formation in north-central Montana. Idem, Feb- 

 ruary, 1915, pp. 95-153. 



22 B. J. Hancock : Geology and oil and gas prospects of the Lake Basin field, Montana. 

 IT. S. Geol. Survey Bull. 691, 1919, pp. 101-148. Geology and oil and gas prospects of 

 the Huntley field, Montana. U. S. Geol. Survey Bull. 711, 1920, pp. 105-148. 



23 R. w. Stone and N. R. Calvert : Stratigraphic relations of the Livingston forma- 

 tion, Montana. Economic Geology, vol. 5, 1910, pp. 551-557, 652-669, 741-764. 



