156 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



beds, which form the lower and greater part of the formation, are capped 

 bv the arenaceous limestone, whose variable nature is its chief characteristic. 

 This bed varies from a pure white or gray, rather coarse granular limestone, 

 composed largely of broken fragments of shells, to a pure sandstone that is 

 occasionally conglomeratic. Near the base the bed is always a limestone 

 and contains carbonaceous remains and fossils of characteristic Jurassic 

 type. The indurated nature of this bed makes it a noticeable feature. It 

 forms good exposures and affords a marked horizon which aids in the 

 working out of the geological structure. The strike of N. 60° E., noted at 

 8,150 feet on the slopes, varies rapidly, as the bed shows an anticlinal fold 

 slightly modified by small local flexures. The dip varies from 40° to 55°. 

 The following section was made of the beds exposed at this locality: 



Section in hills west of Snake River. 



Sandstones; soft, fissile, and crumbly. 

 r Black shales. Feet 



I White limestone 5 



Colorado. { Black shales 100 



Sandstones, ripple-marked and cross bedded 50 



. Black carbonaceous shale, poorly exposed 300 



f Sandstone ; soft and fissile 25 



Dakota.. > Limestone and gray and purple clays. 

 *- Sandstones. 



Ellis Limestone ; arenaceous, cross bedded, fossilil'erous. 



The uppermost beds of this section are exposed about a mile south of 

 the mouth of Glade Creek, the beds having a strike of N. 50° W. and 

 dipping 30° N. 



The Montana formation consists largely of yellowish sandstones, 

 differing in this respect from the character prevailing in the Gallatin region. 

 The beds form a small but steeply sloped hill west of Glade Creek, near its 

 mouth, being exposed in a bluff 150 feet high and 500 feet long. This 

 exposure shows a fault, bringing soft, gray, argillaceous shales against 

 yellowish and gray sandstone carrying abundant fossils of Fox Hills types. 

 The latter beds strike N. 10° E. and dip 30° W. 



Higher up the creek flowing past this bluff the stream has cut very 

 soft, thinly bedded, fissile, light-colored sandstones, overlain by sandstones 

 alternating with occasional beds of shale. No fossils were seen at these 

 exposiires. The higher slopes are everywhere covered by the soft sandy 

 debris derived from the sandstone, and no exposures are seen. In general 



