188 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Big Game and Bobcat ridges and Pinyon Peak to the north is underlain 

 by sandstone. Over the sandstone occur large areas of Pinyon Peak con- 

 glomerate, and much of the country is strewn with unassorted coarse 

 gravel, derived from the disintegration of the more compact conglomerate. 

 Accumulations of glacial material cover large areas. It is a broken, hilly 

 country, with great diversity of topographic features, but picturesque and 

 dotted with groves of scattered pine. It is fairly well watered by numerous 

 small streams, but the gravel deposits are usually dry. 



BIG GAME RIDGE. 



Big Game Ridge is a narrow mountain uplift about 15 miles in length, 

 and rises abruptly above the valley of Pacific Creek along* the southern 

 limit of the forest reservation, with a trend slightly west of north as far as 

 the slopes of Mount Hancock. From the latter mountain, the culminating 

 point, the trend of the ridge changes to northwest, gradually falling away 

 toward the open valley near the junction of Heart and Snake rivers. The 

 eastern boundary of the ridge as far as Crooked Creek is defined by the 

 Snake River fault, which approximately coincides with the course of Mink 

 and Fox creeks, the fault crossing the narrow divide separating- the two 

 streams. From Crooked Creek the deeply trenched but narrow valley of 

 Snake River defines Big Game Ridge from Chicken Ridge. 



Geologically Big Game Ridge is formed mainly of Cretaceous sand- 

 stones, singularly uniform in color and texture from one end of the ridge 

 to the other. They have been referred to the Montana and Laramie forma- 

 tions. In broad masses at certain localities the two formations ma} T readily 

 be distinguished by their lithological habit, but they resemble each other 

 so closely near their junction that any line of demarcation must of necessity 

 be drawn somewhat arbitrarily. In the great thickness of Montana sedi- 

 ments developed here, coarse yellowish-gray sandstones are everywhere 

 the prevailing rock, and nowhere has the Pierre shale been recognized as 

 such by its lithological habit. Evidences derived from organic remains 

 are entirely wanting. Rhyolite skirts the ridge in a number of places, and, 

 as described farther on, caps the very summit of Mount Hancock, and 

 basic breccias cover the lower slopes north of Harebell Creek. 



Gravel Peak. — Tliis peak is situated 3 miles north of the southern limit 

 of the mapped area, midway between Gravel and Mink creeks. It has an 



