﻿Bulletin 



86 



Section at Threeforks, Montana 



Feet 



Feet 



575 ^Jaspery limestones 

 Carboniferous Madison limestone -l 350 Massive limestones 



325 Laminated limestones . . 1250 

 70 Upper Shales 

 Devonian Threeforks shales ■{ (Limestone bed shown in section) 



65 Lower shales 135 



Silurian (?) Jefferson limestones 640 Black limestones 640 



145 Pebbly limestones 

 30 Dry Creek shales 

 Cambrian Gallatin formation ^ 260 Mottled limestones 



J 280 Obolella shales 

 ^120 Trilobite limestone 835 



The composition of the Threeforks shale, which separates the 

 Jefferson limestone from the Madison, is shown in the following 

 detailed section of these beds taken opposite Logan : 



Section of Threeforks shales at Logan, Montana 



Feet 

 E. Buff or yellowish calcareous sandstone, becoming a shale in lower 



half 30 



D. Gray limestone and shale ^. . 5 



C. Green argillaceous shale, with limestone concretions and an 



abundant fauna 50 



B. Gray limestone, huffish at base 10 



A. Buff, locally reddish, sandy shale, with shaly yellow limestone 



near top 45 



140 



The limestone separating the Upper and Lower shales of Peale's 

 section is represented by B of the Logan section. Through the 

 expansion of this bed or the interpolation of others the Three- 

 forks shale sometimes becomes more of a limestone than a shale 

 formation which has led to the use of the name Threeforks lime- 

 stone instead of Threeforks shale in some of the publications of the 

 Survey. 



While the Jefferson limestone is generally nearly black or dark 

 chocolate colored, beds • of mottled buffish color are in places 



