BIGHORN PASS. 25 



the Park. The pass is cut in the Paleozoic sedimentary rocks, which are 

 slightly tilted by an intruded sheet of andesite-porphyry that is the 

 northern extension of the Indian Creek laccolith. A dark, lamprophyric 

 rock occurs at the lowest point in the pass, where it is seen to form a sheet 

 50 feet thick intrusive in the Cambrian shales. The high ridge extending 

 north from the head of Indian Creek to Bighorn Pass is formed of sedi- 

 mentary beds that overlie the northward extension of the Indian Creek 

 laccolith. Above the andesite-porphyry of the laccolith which forms the 

 Indian Creek Pass the green Flathead shales are exposed, overlain by 

 the upper limestone series of the Flathead, which are here 100 feet thick 

 and resemble quite closely the beds of this horizon as developed in Crow- 

 foot Ridge. The summit of the ridge is formed of the Gallatin " mottled 

 limestone," which dips to the northwest and makes a well-defined ledge, 

 with a cliff face 30 feet high and a rounded but hummocky surface, the 

 result of g-lacial planing. Near Bighorn Pass the beds are locally affected 

 by an intrusion of the laccolith, and dip more steeply than the beds north 

 of the pass, there being a difference of 5° to 8°. Two sections of the 

 Paleozoic rocks were measured in this vicinity; the first was made on the 

 ridge running south, the second from Bighorn Pass to the summit of 

 Bannock Peak. These sections show the following* sequence of beds, 

 arranged in descending order: 



Bighorn Pass section. 



Crowfoot Nura- 

 section. ber. Feet. 





22 Cherty sandstones and limestones, brown and gray, chert nodules, with 

 I fi I whitened surface 75 



) 



21 Quartzose conglomerate ; matrix a light-gray limestone 20 



f 33 20 White beds of sandstone, quartzite, and limestone, generally saecharoidal 325 



©J 



'32 19 Limestone, crystalline, light colored. The upper portion is a brecciated and 

 nearly pure limestone. The lower beds are dense, finely crystalline, weath- 

 ering white, with granular surface. Maguesian, and containing sparsely 



disseminated chert. Strike, S. 30° W. ; dip, 10° N\V 275 



32 18 Limestone, very finely crystalline, with calcite strings, brown, dense, massive; 



nearly pure 25 



31 17 Limestone, breccia; the matrix is similar to the bed below; the fragments 

 resemble the beds above and below. The lower beds are slightly siliceous, 



light-gray limestones weathering pale yellow , 50 



31 16 Limestone, crystalline, light brown, with granular weathered surface; mass- 

 ively bedded, and is vertically jointed and contains some chert in bands 75 



■a 



