WYOMING ROCKIES 



371 



northwest of the park. The Washburn Range within the park is formed 

 entirely of the early basic breccias. 



A trachyte was possibly extruded next, approximately along the course 

 of the Yellowstone River. Then over the early basic breccias, but nowhere 

 over the trachyte, were poured out a great series of basalt flows 1200 feet 

 thick. These basalts form many of the higher flat-top summits in the 

 northern part of the Absaroka Range, but are overlain by later deposits to 

 the south. They are distributed widely in eastern Yellowstone Park, and 

 they are an important horizon marker because they separate the early 

 breccias from the later. 



Renewed explosive activity resulted in the accumulation of the late acid 

 breccias previously mentioned. They are limited chiefly to those portions 

 of the Absaroka Range that lie within the Park and extend westward to 

 Yellowstone Lake. 



A period of erosion evidently followed, and the late basic breccias 2500 

 feet thick were deposited over an irregidar surface. They form extensive 

 plateau areas in southeastern Yellowstone Park and make up chiefly the 

 southern half of the Absaroka Range but are exposed sparingly over the 

 late acid breccias. Where the late acid breccias are absent, the late basic 

 breccias rest directly on the early basalt sheets. The eruption of more 

 basalt flows closed the period of late basic breccia volcanic activity. 



The last of the breccia series, which includes the early and late breccias 

 and the basalts, was an andesite outpouring in the southeastern part of 

 the park. It is now preserved in the higher peaks there. 



The following is Howard's summary ( 1937 ) of the post-breccia history 

 of Yellowstone Park. 



Post-Breccia Faulting. Study of the Washburn Range indicates that the 

 next event of major importance was extensive faulting of the great series of 

 volcanic rocks previously described. It appears to have been this faulting that 

 gave the Washburn Range a relief so great that it could not be buried by the 

 later rhyolite floods. Presumably, other inequalities of the old basin floor are 

 attributable to faulting at this period, but only where the later rhyolite failed to 

 bury the inequalities, or where post-rhyolite erosion has later uncovered them, 

 1J can the evidence of faulting be studied. 



Post-Breccia Erosion. The faulting of the great masses of pre-rhyolite vol- 

 canic formations was associated with a long period of erosion, sufficiently im- 

 portant to deserve special mention. Locally, at least, a gently rolling topography 



was developed on the breccias within the park. Thus, the erosion contact be- 

 tween the breccias and the overlying rhyolite, where exposed for a distance of 

 8 miles in the walls of Yellowstone Canyon, from Broad Creek almost to Tower 

 Creek, is gendy undulating. Elsewhere, the relief is much greater, but how 

 much of it is due to faulting is unknown. Presumably, the faulting took place 

 progressively over a considerable period, and erosion must have accompanied 

 the movements. Whether sufficient erosion preceded the faulting to produce a 

 faint relief, which was then locally intensified by uplift, or whether strong relief 

 due to early faulting was not effaced by the erosion that elsewhere produced a 

 gently rolling topography, is not clear from the evidence obtained. 



After the faulting and erosion, the Yellowstone basin, its dissected sides and 

 floor now composed partly of pre-Tertiary rocks of all kinds and, partly, of 

 Tertiary volcanic rocks, received the floods of late Tertiary rhyolite lavas. The 

 rhyolite floods were locally preceded by basaltic extrusions. 



Early Canyon Basalts. Basalts are found locally under the rhyolite, below 

 the level of the surrounding breccias, and indicate a period of eruption later 

 than that represented by the basalt that closed the period of breccia accumula- 

 tion. The early Canyon Basalts were probably poured out after the faulting of 

 the breccias and after the extended erosion period associated with that faulting. 

 They are exposed in patches along Yellowstone Canyon and in the canyon of 

 Gardiner River, in the northwestern part of the park. 



Rhyolite Floods. There now occurred one of the most remarkable events in 

 the history of Yellowstone Park, for enormous floods of rhyolite lava filled the 

 lowlands of the earlier landscape to depths of a thousand feet or more, swept 

 around the Washburn Range and other highland areas, which stood as islands 

 in the lava sea, lapped against the foothills of the encircling ranges, and con- 

 tinued an unknown distance to the west, where the mountain rim is lacking. 

 Today, the lava plateau terminates a short distance outside the Park in a steep 

 scarp of uncertain origin, which drops sharply to the lower Snake River Plains. 



Certain basalts in the northern part of the Park, at the edge of the basin, may 

 have been extruded during pauses in the extrusion of the rhyolite. 



Post-Rhyolite Faulting. Following die extrusion of the rhyolite, the broad, 

 level plateau surface was broken by block-faulting, perhaps a result of setding 

 in response to the withdrawal of the vast quantities of magma from below. 

 Many of the lake and hot-spring basins, and many of the "topographic fault 

 blocks" visible on the contour maps, may have been formed at this time. 



Post-Rhyolite Erosion. There next ensued a period of erosion, the extent of 

 which remains an unsolved problem. The crispness of most of the block units 

 in the topography, however, suggests slight denudation of the park area as a 

 whole, but a few deep valleys, such as the Lamar Valley in the north, may have 

 been eroded. The carving of the Grand Canyon of the Yellowstone River may 

 have begun at this time or during the first half of the Pleistocene. Its present 

 depth, however, was attained during the late Pleistocene. The scarp th.it limits 

 the rhvolite plateau to the west was presumably fashioned at this time, for its 



