166 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



outline is in great part due to the sinuous border of accumulated lavas that 

 abut against the steep slopes of uplifted strata. Across its broadest expan- 

 sion, from Snake River to Two Ocean Plateau, it measures about 20 miles. 

 In length it measures nearly 22 miles, stretching northward with decreasing 

 breadth across the forest reservation, gradually dying out in a narrow ridge 

 projecting- into the rhyolite body which skirts the west shore of the south 

 arm of Yellowstone Lake. 



All of the ridges and mountain masses which make up this region 

 are clearly defined by salient topographic features, delimited by deeply 

 eroded valleys, and yet they are all so knit together by outlying spurs and 

 elevated passes as to present a single mountain group, with a somewhat 

 complex topographic structure and an intricate drainage system. The 

 principal physical features are Big Game Ridge and its extension north- 

 ward, Chicken Ridge; Piny on Peak and Bobcat Ridge; Huckleberry 

 Mountain and Wildcat Peak. Several of the high mountains of the Park 

 country are found here, a number of them attaining elevations of over 

 9,500 feet, but only one, Mount Hancock, the culminating point of Big 

 Game Ridge, reaches an altitude of over 10,000 feet above sea level. 

 With the exception of a narrow strip of country pouring its waters into 

 Yellowstone Lake, this entire region is drained by Snake River or some of 

 its many tributary streams. The main branch of Snake River takes its rise 

 along the west slopes of Two Ocean Plateau, flows northerly around the 

 east base of Mount Hancock, and thence, with a sharp curve around the end 

 of Big Game Ridge, runs southerly at the west base of the same mountain. 

 With a gentle sweeping curve it encircles the northern end of Huckleberry 

 Mountain and enters the broad, open plain lying west of this mountainous 

 region. Continuing its course southward, it crosses the forest reserva- 

 tion, and soon after widens out into Jackson Lake, a short distance south 

 of the limit of the map. With this circuitous coui'se, as thus defined, Snake 

 River nearly surrounds this group of mountains, and on leaving the forest 

 reservation has become a wide, deep stream. Across the broadest expanse 

 of these mountains there runs, in an approximately east-west line, a well- 

 defined watershed from Snake River to Two Ocean Plateau. North of 

 this divide such fine streams as Coulter, Wolverine, and Fox creeks pour 

 large volumes of water into the main Snake. Several mountain torrents, 

 notably Lizard, Gravel, and Mink creeks, flow southward into Pacific 



