COULTER CREEK. 179 



REGION OF COULTER CREEK AND BOBCAT RIDGE. 



coulter creek. — Coulter Creek, before emptying into Snake River, flows 

 through an open basin, and is greatly augmented by the drainage of two 

 such large streams as Harebell and Wolverine creeks. All these streams 

 have brought down large accumulations of drift material, and the underlying 

 sedimentary rocks are for the most part obscured by a thick mantle of 

 gravel, sand, and fine breccia. The mountain slopes surrounding this little 

 basin are well terraced, and five sharply defined benches rise one above the 

 other to a height of nearly 200 feet. Coulter, Harebell, and Wolverine 

 creeks all enter this basin through narrow defiles cut in breccia, with 

 perpendicular walls 300 feet in height in places. Coulter Creek, for nearl}' 

 4 miles, has cut its way through this breccia, and it is practically impassable 

 from the number and size of the bowlders which lie piled up along the 

 valley. From beneath this dark, somber breccia Cretaceous sandstones and 

 shales crop out at a number of localities and on the west side of Coulter 

 Creek extend along the base of the mountain, with the breccias lying above. 



The main tributary of Coulter Creek from the west has cut through 

 the breccia, and erosion has carried it away from the mountain slopes so as 

 to expose Montana sandstones all the way from Huckleberry Mountain to 

 Coulter Creek. In the region of Coulter Creek the undertying sandstones 

 possess a very irregular surface, and in many places the breccia rests upon 

 them as a thin flow or crust. The sandstones show a prevailing dip to the 

 southwest. This body of breccia stands by itself, completely isolated from 

 the vast pile of breccias of Two Ocean Plateau and the Absaroka Range. 

 It is very irregular in outline and measures about 8 miles in length, 

 stretching' from the source of Coulter Creek northward along- the west 

 slopes of Mount Hancock. 



On the east side of Coulter Creek the breccia attains an elevation of 

 8,500 feet, and on the slopes of Mount Hancock reaches nearly the same 

 elevation. Throughout the entire body the breccia presents much the same 

 general habit, a coarse, firmly compacted agglomerate, dark brown or black 

 in color, with a lighter-colored cementing material. Some portions of it are 

 exceptionally coarse. Fragments of well-rounded gneiss and quartz have 

 been observed embedded in the mass. In general it is free from evidence 

 of bedding, as is well shown in the exposures along the main stream. At 



