466 KEMP AND BILLINGSLEY SWEET GRASS HILLS, MONTANA 



The formations found in the Hills, with their thicknesses, are sum- 

 marized below: 



Pleistocene Glacial moraines. 



Glacial till. Feet 



Upper Cretaceous 



Judith River . . top eroded 



Montana group J Claggett 500 



1 Eagle 150 



^, , , (Upper member. 1,000 



Colorado group. . . .J , .-,„„ 



) Ivower member . 800 



Lower Cretaceous . . . Kootenai formation 450 



Jurassic Ellis formation 200 



Unconformity. 



Mississippian Madison limestone base not exposed 



These will be discussed in order, from oldest to youngest. 



Madison limestone.- — The top of this widespread and vmiform Carbon- 

 iferous limestone is exposed on the north, east, and south slopes of East 

 Butte. It lies at steep quaquaversal dips, being apparently domed up 

 by a concealed laccolith, but also bears upon its upper surface the large 

 laccolith which appears on the map. With intrusive rocks above and be- 

 low, the formation is naturally considerably metamorphosed, and marble, 

 magnetite, garnet zones, etcetera, occur at several points. 



The Madison, so far as seen, consists of heavily bedded blue-gray 

 limestone, with some lighter members bedded in layers about 2 feet thick. 

 Since the top of the formation is an erosion surface and the base is not 

 exposed, the thickness can not be determined, nor can even the relative 

 place in the formation of the exposed section. 



Overlying unconformity. — Normally, the Pennsylvanian, Permian, 

 Triassic, and Jurassic formations should come above the Mississippian. 

 In the Sweet Grass Hills is a gap extending to the Jurassic^ This is 

 not true of the greater part of Montana. In the Pryor Mountains, near 

 Billings, above the Madison limestone, are Amsden red shales (Pennsyl- 

 vanian), Tensleep sandstone (Pennsylvanian), Eml)ar limestone (chiefly 

 Permian), Chugwater red beds (Permian and Triassic), and Sundance 

 (Ellis) limestone and sandstone (Jurassic). The strata between the 

 Madison and Ellis have an aggregate thickness of about 1,200 feet. 



Investigations in central Montana show that between the Pryor Moun- 

 tains and the Sweet Grass Hills there is a steady change in the character 

 and in the thickness, of these intervening formations. The changes in 



