218 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



valleys of the streams emptying into it from the south. It also extends 

 down the valley of Soda Butte Creek near to the mouth of Amphitheatre 

 Creek, and disconnected areas of it occur at the mouth of Pebble Creek 

 and near Soda Butte, as described in the previous chapter. It also forms 

 the head of Pebble Creek and the valley of Slough Creek, a small portion 

 of which is shown in the northwestern corner of the map. The limestone 

 embraces the Cambrian, Silurian, and Devonian, which attain a thickness 

 of only 1,800 feet, the strata of the last two periods being very poorly 

 developed and not exceeding- in thickness 400 feet. In most places the 

 limestone extends up into the Carboniferous. The whole series is very 

 poor in fossils within the area of the map, and the identification of the 

 horizons rests on evidences obtained from neighboring sources. 



The very irregular form of the eroded surface of the limestone is shown 

 by the variable heights of the limestone cliffs, which range from 400 to 

 2,400 feet. The highest within the area are in the peak southwest of Cook 

 City, in Hunter Peak, and in the escarpment south of the mouth of Cran- 

 dall Creek. The extensive erosion which preceded the ejection of the lavas 

 was subsequent to the orographic movement that involved the coal-bearing 

 Laramie strata in this region. Upon this greatly eroded surface were 

 thrown volcanic tuffs and scoria, with occasional streams of lava, until the 

 accumulation exceeded in thickness 4,000 feet. The breccias were traversed 

 in various directions by dikes of lava which filled the crevices made during 

 the later eruptions. 



The close of volcanic activity in the vicinity of Crandall Basin was 

 followed by another period of erosion which removed the upper portion of 

 the volcanic accumulation, leaving over 4,000 feet of it in the form of high 

 ridges and peaks, with deeply cut valleys between. The occurrence of 

 nearly horizontal layers of massive lava alternating with crudely bedded 

 tuffs and breccia, which in places are piled one on another to the number 

 of twenty or more, gives rise to precipitous mountains, which are quite 

 inaccessible from most points of approach. The highest of these is Index 

 Peak, whose steepledike summit, 11,700 feet in altitude, has not yet been 

 scaled. A view of this peak from Clark Fork gives an idea of its sharpness. 

 (PI. XXVIII.) The readiness with which the scoriaceous breccia and tuffs 

 are eroded causes the drainage channels to be narrow and rocky, so that the 

 valleys traversing this region are in general very difficult to travel through. 



