176 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



To the east of this depression the sandstones assigned to the Dakota 

 formation are clearly defined along the west side of the prominent ridge 

 separating Red and Basin creeks. They pass gradually, without any sharp 

 line of demarcation, over into the Ellis beds, which form the summit of 

 the ridge. The axis of the anticline lies in the Ellis limestone, which is 

 here nearly 200 feet in thickness, and is characterized by a small but well- 

 defined Jurassic fauna. Among the species found here are Gryplicea calceola 

 var. nebrascensis, Camptonccies pertenuistriatus, Gervillia, Pseudomonotis curta, 

 MoclioJa suhimbricata, Ammonites. 



At the east base of the ridge and on the opposite side of the anticline 

 the Dakota sandstones again come in, dipping eastward until finally lost 

 beneath the rhyolite which skirts the edge of the upturned sedimentary 

 beds. 



East of Basin Creek the valley of the Snake is broad and open, show- 

 ing wide alluvial meadow lands, diversified by occasional growths of pine. 

 A short distance below the mouth of Basin Creek the river flows through a 

 narrow chasm before it enters the wider valley near its junction with Coulter 

 Creek. At the upper entrance of this chasm the river cuts through a ridge 

 of coarse breccia, showing a mural face nearly 300 feet in height. This 

 breccia rests upon Montana sandstones, which form the greater part of the 

 hills on the west side of the river. On the east side of this chasm, about 

 half a mile above the junction of the river with Coulter Creek, occur two 

 outcrops of coal, exposed just above the river bank. These seams of coal 

 along the outcrops measure about 15 inches in thickness. They are over- 

 lain by fine conglomerate and underlain by black arenaceous clay. These 

 coals are more or less impure by reason of thin bands of carbonaceous 

 clay. The beds carrying the coal strike N. 30° W. and dip from 15° to 

 20° E. The bedded sandstones carrying the coal seams pass under the 

 river, but are not exposed on the opposite side, owing to the accumulation 

 of glacial drift. Below the junction of Coulter Creek with the Snake the 

 sandstones dip to the northeast, away from the river, while, as already 

 noted, they dip to the southwest on the opposite side of the stream. 



Notwithstanding the occurrence of coal these sandstones are regarded 

 as belonging to the Montana formation, and probably to the upper part, or 

 the Fox Hill terrane. Owing to the great uniformity in the sedimentation 



