338 GEOLOGICAL EXCLUSION TO THE ROCKY MOUNTAINS. 



gneisses and schists, probably of Archean age, flanked on the northern 



spurs by uplifted Paleozoic rocks. 



Bast of the Tetons, across the broad valley of the Upper Snake, 

 generally known as Jackson Basin, lies the well known Wind River 

 range, fatuous from the earliestdays of Rocky Mountain trappers. The 

 northern end of this range is largely composed of Mesozoic strata, single 

 ridges of upper Cretaceous sandstone penetrating still further north 

 into the regions of the Park until buried beneath massive Hows of lava. 



Along the entire east side of the Park stretches the Absaroka range, 

 so called from the Indian name of the Crow Nation. The range is in- 

 timately connected with the Wind River range, the two being so closely 

 related that any line of separation must be drawn more or less arbi- 

 trarily, based more upon geological structure and forms of erosion than 

 upon any clearly denned physical limitations. The Absarokas stretch 

 for more than SO miles, a rugged, unbroken mountain mass, without any 

 good pass across them. They have always stood as a formidable barrier 

 to all western progress, and to-day are only crossed by hunters and 

 mountaineers by one or two dangerous trails known to but few. All 

 the upper portion of the range is formed of eruptive rocks that have 

 poured out in such enormous masses as to conceal an earlier range made 

 up largely of Mesozoic and Paleozoic strata, extending from the Cam- 

 brian to the Upper Cretaceous. The latter are seen all along the east 

 base of the range, and at the northern end in Chirks Fork valley, and 

 in the Park at the junction of Soda Butte creek and the Lamar river. 



At the northeast corner of the Park a confused mass of mountains 

 connects the Absarokas with the Snowy range. This latter range 

 shuts in the park on the north, and is an equally rough region of coun- 

 try, with elevated mountain masses covered with snow the greater part 

 of the year, as the name would indicate. Only the southern slopes, 

 which rim in the Park, bear upon the geology of the region. Here the 

 rocks are mainly granites, gneisses and schists, with sedimentary beds, 

 for the most part, referable to pre-Cambrian series. They are in great 

 part overlain by Tertiary lavas. 



The region has been one of profound dynamic action and a center of 

 mountain building on a grand scale. So far as the age of these moun- 

 tains is concerned, evidence goes to show that upheaval was contem- 

 poraneous in all of them, and coincident with powerful dynamic influ- 

 ences which uplifted the north and south ranges stretching across Colo- 

 rado, Wyoming, and Montana. These dynamic movements blocked 

 out for the most part the Pocky Mountains near the close of the Ore 

 taceous, although there is good reason to believe that in the region of 

 the Park profound faulting and displacement continued the work of 

 mountain building into much later time. By the building up of these 

 mountains a depressed basin was formed, everywhere inclosed by high 



