.46 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The following section represents the beds exposed on the northern slopes 

 of this ridge at a point just west of the 9,900-foot peak: 



Section at ridge southwest of Gray Peak. 



Num- 

 ber. Feet. 



3 Dakota conglomerate and sandstone; pebbly in layers, but much of it a fine-grained sand- 

 stone, buff with red and white blotches, and cross bedded 60 



2 Shaly beds, very arenaceous and light brown at top; softer and more argillaceous below, 

 where the layers are generally a blue-black and contain some splintery limestone. The 



typical rock is a soft argillaceous sandstone, light gray, weathering brown 50 



Andesite-porphyry, dark gray, compact; rock occurring irregularly columnar in ledges, 



with two or three layers of brown altered porphyry with fine concentric weathering.. .. 35 



1 Limestone, gray with rusty speckling, saccharoidal texture. The upper 8 feet soft and 



purple shale, weathering brown 18 



Strike, N. 20° E. ; dip, 8°-10° W. 



On the west side of this peak, 100 feet below the summit, the black 

 shales are well exposed and form the crest of the ridge down to the saddle. 

 West of the saddle they give place to the Dakota sandstone, which extends 

 westward to the cliffs indicated upon the map. Farther west the ridge 

 shows a succession of light-gray limestones overlying the red Teton sand- 

 stones. The latter beds form red slopes that extend westward to the 

 andesite-porphyry hill shown upon the map. 



The central ridge of the Fan, lying north of Stellaria Creek and west 

 of Joseph Peak, is a long flat-topped mass with grassy meadows and dense 

 forests of pine. On the western end of this ridge exposures are scarce and 

 must be sought for in the stream channel. A short distance above the forks 

 of Fan Creek, Stellaria Creek has cut a gorge through the intrusive sheet 

 of andesite-porphyry. This rock is also exposed on the south slopes of the 

 ridge to the north for a distance of 2^ miles above the mouth of Stellaria 

 Creek. The rock is generally much decomposed, of a light-buff color, 

 with numerous decomposed acicular hornblende and white plagioclase 

 phenocrysts. The porphyry forms great heaps of tabular debris, often 

 arranged in ridges running approximately east and west and separated from 

 the solid rock by the hollows between the cliffs and these morainal ridges. 

 These hollows are often without outlet, and sometimes hold small ponds. 

 From the junction of the stream eastward the summit of the ridge shows 

 no outcrops of sedimentary rocks, the covering of the porphyry sheet just 

 noticed having been removed by erosion and the summit being now heavily 

 mantled with glacial drift, which seems to be at least 100 feet thick in the 



