42 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



of the rock. A well-marked bench occurs along the top of the steep slope 

 or escarpment, its surface in places sloping with the dip of the strata. It is 

 most pronounced when above the harder beds, especially where these are 

 overlain by the more friable sandstone and soft shales of the Juratrias 

 formations. 



The bench is well developed on both sides of the drainage channel 

 running west from Fawn Pass. Upon entering the terrane of the Juratrias 

 formations, which are mainly fissile limestones and shales, passing down- 

 ward into the sandstones of possible Triassic age, and passing upward into 

 the friable sandstones underlying the Dakota conglomerate,' we again 

 encounter intruded sheets of igneous rock. These intrusive sheets are 

 nearly conformable with the sedimentary strata, following the shale belts 

 for long distances, and only occasionally breaking up across the strata 

 to follow higher horizons. The first of these intrusive sheets met with 

 above the Carboniferous limestones occurs in the base of the Juratrias 

 shales below the Ellis shale beds, and forms a cliff rising above the bench 

 west of Fawn Pass. The sheet of andesite-porphyry is possibly 200 feet 

 thick at this place. It can be traced west and east from this exposure, con- 

 tinuing at nearly the same horizon. The shale in contact with the porphyry 

 is more or less baked, and, like the igneous rock, resists erosion better 

 than the altered shales, thus leading to the formation of bluffs or ledges. 



Above this porphyry cliff the country slopes gradually and stretches 

 eastward to Fawn Pass, rising abruptly to the triangular peak 1^ miles 

 southwest of Gray Peak. This comparatively level country occupies the 

 axis of a flat synclinal arch that dips to the northwest. The strata bend 

 around from the gentle northeast dip through a northwesterly one to a 

 southwest dip of 15° to 20°, in which position they form the triangular 

 peak just mentioned. It is evident, from a study of the region, that this 

 southwesterly dip is due to the intrusion of a large body of igneous rock 

 connected with the bysmalith of Gray Peak. The highest sedimentary 

 rock in the triangular peak is Dakota conglomerate. It occurs again high 

 up on the west spur of Gray Peak, where it dips toward the northeast. 

 The ridge between these points traverses an anticlinal arch of Jurassic beds 

 that bend over the igneous mass already mentioned. The shales include at 

 least two thin sheets of igneous rock, each from 40 to 100 feet thick. One 

 of these, in the mass of the triangular peak, thins out perceptibly toward the 



