GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 459 



on the succession of beds, but on ripple-marks and cross-bedding, 

 and is beyond dispute."^ 



The maximum thickness of the formations displaced by this 

 fault is not less than 10,000 feet, and in places may be considerably 

 more. The actual fault plane has nowhere been observed, but it 

 dips westward at an angle apparently higher than is characteristic 

 of most other Rocky Mountain overthrusts. The strata east of 

 the fault are in some places sharply flexed whereas in others they 



Fig. II. — The Beartooth fault zone southeast of Stillwater Valley. Vertical 

 Madison limestone along the range front, with gently east-dipping Livingston beds on 

 valley slope. Looking southeast. 



dip gently away from the fault zone (Fig. 11). At Red Lodge the 

 Fort Union formation dips 18° toward the mountain front, but in 

 the intervening distance of 4 miles it becomes horizontal, then dips 

 gently in the opposite direction as the fault is approached. 



The Beartooth overthrust appears to be the northward extension 

 of the Heart Mountain overthrust of northwestern Wyoming, which 

 has been determined recently by Hewett to have an eastward 

 thrust of not less than 28 miles.^ If not the direct continuation of 

 the Heart Mountain fault it was very probably formed during the 



^ C. L. Dake, letter to the writer, Feb. 13, 1921. 



= D. F. Hewett, "The Heart Mountain Overthrust, Wyoming," Jour. Geol., 

 XXVni (1920), 536. 



