192 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the south arm of the lake. The deeply trenched valley of Snake River 

 limits the mountains on the south and southwest, and the rhyolites of the 

 Park Plateau, stretching- from Heart River to Yellowstone Lake, submerge 

 between an accumulation of lava the western flanks of the mountains to 

 heights varying from 8,000 to 8,500 feet. 



Topographically Chicken Ridge is closely related to Big Grame Ridge, 

 the latter trending off to the northwest, while the former hag a nearly 

 north-south course. Geologically the relationship is still closer, and the 

 Cretaceous strata of Big Grame Ridge can be easily traced crossing the river 

 at several localities. This is specially well shown at the southern end of 

 Barlow Peak, just north of Crooked Creek 



Banow Peak. — Barlow Peak is named by the writer in honor of Maj. J. W. 

 Barlow, of the Engineer Corps of the Army. He conducted the first official 

 exploration to the headwaters of Snake River in 1871. The peak attains 

 an elevation of 9,500 above sea level, and rises 1,500 feet above the river. 

 It forms a well-defined mountain block between Crooked and Sickle creeks, 

 the former stream encircling its southern base, and the latter cutting a deep 

 trench directly across Chicken Ridge, flowing into Suake River 4 miles 

 farther downstream. 



Just below the mouth of Crooked Creek, beds assigned to the Laramie 

 are found on both sides of Snake River, dipping to the southeast and east. 

 They cross the spur of the mountain, continuing eastward until lost in the 

 accumulation of drift, and are finally cut off by the Snake River fault. The 

 Laramie sandstones reach nearly to the summit, and along the crest of the 

 ridge are underlain by yellowish-gray sandstones similar to those found high 

 up on the slopes of Mount Hancock. The Montana sandstones cap the 

 summit of Barlow Peak, and along the east slope dip 10° to 15° E. From 

 this point they can be traced northward across Sickle Creek, still inclined 

 in the same direction. 



Passing down Sickle Creek the underlying beds gradually grade into 

 thinly bedded sandstones, limestones, black shales, and argillaceous sand- 

 stones, and are well exposed on both sides of Snake River. Lithologically 

 these beds bear the closest resemblance to the sedimentation of the Colorado 

 formation found elsewhere. The thickness of these black clay shales and 

 interbedded sandstones has been estimated at 600 feet. On the north bank 

 of Snake River, about a quarter of a mile above the mouth of Sickle 



