Haynes: Fauna of the Upper Devonian in Montana. 17 



Section D, measured two miles farther north is as follows: 



Base of gray Madison limestone. 



1. Yellow sandstone with some shale 44 feet. 



2. Yellow shaly limestone 15 



2a. Yellow argillaceous shale 5.5 



3. Purplish red shale i 



4. Nodular gray limestone 8 



5. Green shale 49 



6. Orange limestone | 



and > 130 



7. Reddish yellow shales mostly obscured J 

 Top of Jefferson limestone. 



Total 252.5 feet. 



The region north of Three Forks and west of the Missouri River 

 has many good exposures of the Three Forks Formation. The valleys 

 eroded in the formation have a general north to south direction and 

 are nearly parallel with one another. The repetition of the formation 

 is due partly to folding and partly to faulting. The structure in the 

 central part of the ridge is that of a southward pitching anticlinal fold 

 which is overturned to the east. A very large valley is eroded in the 

 Three Forks formation in the western limb of the fold. The strata 

 here strike N. 10° E. and dip 30° W. The following section was 

 measured on the western side of this valley across the upper part of 

 Three Forks Formation. 



Sect i 071 E : 



Base of gray Madison limestone. 



I and 2. Yellow sandstone and shale 75 feet. 



3 and 4. Purplish shale with limestone at the base 20 



5. Fissile green shale with layers of limestone concretions. 

 Lower part of section obscured. 



West of the overthrust fault there is another valley formed in the 



Three Forks Formation. The strata here strike N. 20°-30° E. and dip 



30°-40° W. The best exposures were seen on the western side of 



the valley in the small tributary gullies which cut across the dip of 



the strata. Partial sections were measured along this valley at several 



places in a distance of over two miles. These sections, beginning at 



the northern end of the valley, are as follows: 



Section F, on tributary gully 3, western side of valley. 



» 

 Base of yellow shale No. 2. 



3. Purplish black shale weathering reddish 13-5 feet. 



4. Gray limestone 5 



Top of green shale No. 5. 



