TWO OCEAN PLATEAU. 201 



stands out as a prominent and somewhat isolated physical feature, sepa- 

 rated from the Absarokas by the wide, flat valley of the upper Yellowstone, 

 and sharply denned on the north by the broad sheet of water known as 

 Yellowstone Lake. The continental divide crosses the summit of the 

 plateau with a northeast-southwest course, sending its waters either to the 

 Yellowstone and the Atlantic or the Snake and the Pacific. Owing to its 

 broad mass and great elevation the snows of winter accumulate upon it to 

 great depths, and the rains of summer furnish an abundant water supply. 

 Numerous streams leaving the plateau have trenched deep, narrow lateral 

 canyons into the pile of breccias, which, with steep mural faces, drop 

 abruptly for 2,000 feet to Yellowstone River. 



The plateau presents a comparatively uniform mass of basic breccia 

 throughout its entire length, from Two Ocean Pass, which defines its 

 southern limit, northward to Yellowstone Lake. Andesitic and basaltic 

 fragments, more or less firmly compacted together by fine cementing mate- 

 rial, make up the greater part of the mass. In most respects it is quite like 

 the basic breccias described elsewhere as occurring all along the west slopes 

 of the Absarokas. Exceptionally fine exposures of the mass are shown in the 

 abrupt escarpments along the upper Yellowstone all the way from Atlantic 

 Creek northward to Badger Creek. Seen westward across Yellowstone 

 Valley these walls are specially impressive, and are easily studied along 

 the cliffs of Falcon and Lynx creeks. Interbedded in the breccias are 

 occasional flows of compact basalt, lying in beds one above another. They 

 usually rest upon an irregular, uneven surface of the breccia, and, from 

 evidences of weathering observed in the disintegrating breccias, indicate 

 clearly that these basalts were poured out over a surface exposed to atmos- 

 pheric agencies for a long period of time. The varying thickness of the 

 basalts, their thickening and thinning, and their entire absence over extended 

 areas prove how irregular and intermittent were the overflows. They are 

 by no means so thick or so persistent as the basalt sheets observed in the 

 earlier basic breccias in the main portion of the range. The heaviest 

 developments of these interbedded flows are seen in the southern end 

 of the plateau, but they seldom attain a thickness of 100 feet, and in 

 such instances are made up of a series of individual flows not always 

 continuous. 



While the great body of basic breccia consists of coarse angular 

 fragments, with here and there basaltic bowlders measuring 2 feet in 



