ENVIRONMENT OF THE TITANOTHEKES 



51 



CHARACTER OF THE MOUNTAIN-BASIN, PLATEAU, AND 

 PLAINS REGIONS 



The geographic history of the mountain-basin 

 region and of the Plains region presents some resem- 

 blances and some contrasts. Both regions were 

 subject to slowly progressive elevation during this 

 period. Nearly all the Eocene deposits of the moun- 

 tain basins were laid down in broad, fiat valleys and 

 on mountain plateaus, which were drained largely 

 by the same great river systems that drain them 

 to-day, whereas those of the Plains region were 

 widely scattered over broad flood-plain areas in 

 which the rivers frequently changed their courses, the 

 present river courses being cornparatively modern. 

 In the mountain basins, from the basal Eocene of 

 the Fort Union, Puerco, and Torrejon formations to 

 the summit of the upper Oligocene as represented 

 in the John Day formation of the Columbia Plateau, 

 the older Tertiary rocks were at very few places 

 worked over into newer deposits, but at many places 

 deposition was continuous. Despite continuous ero- 

 sion since Oligocene time large areas of the historic 

 Eocene sediments of the mountain-basin region have 

 been preserved in their original purity and con- 

 tinuity for the geologist and paleontologist. By 

 contrast, in the Plains region large areas of the 

 original Oligocene strata were in part worked over 

 to form Miocene strata, and part of these in turn 

 were eroded to form Pliocene strata; again all three 

 contributed to the Pleistocene strata; and finally all 

 four are now contributing to the alluvium of the 

 Great Plains. 



EOCENE TOPOGRAPHY IN THE ROCKY MOUNTAIN REGION, 

 MONTANA TO NORTHERN NEW MEXICO 



By middle Eocene time the topography of the Rocky 

 Mountain region from Montana to northern New 

 Mexico had become broadly similar to that of to-day. 

 The existing sharply sculptured ranges of the Big 

 Horn, Wasatch, Uinta, and San Juan Mountains are 

 remnants of much loftier ranges, which had their 

 birth in late Cretaceous and early Eocene time. The 

 two great drainage systems of the region — (1) Big 

 Horn, Yellowstone, and Missouri Rivers on the north 

 and (2) Green, White, San Juan, and Colorado Rivers 

 on the south — were probably well established at the 

 end of Eocene time. 



According to Ransome (1915.1) and Lindgren 

 (1915.1) the general uplift of the land in the Rocky 

 Mountain region near the end of Cretaceous time 

 was not uniform at different points either in its incep- 

 tion or in its intensity. Apparently the earliest move- 

 ment occurred after the deposition in the Denver 

 Basin of the conformable series of Cretaceous beds 

 that is now called the Laramie formation, which over- 

 hes the Fox Hills sandstone. The Front Range of 

 central Colorado arose at this time, before the deposi- 



tion of the Arapahoe formation of Colorado (Ran- 

 some, 1915.1, p. 361). Andesitic tuffs and flows occur 

 in the Denver formation, which immediately overlies 

 the Arapahoe. At the south end of the Rocky Moun- 

 tains, in northern New Mexico, great uphfts occurred 

 both before and after the deposition of the basal 

 Eocene Puerco and Torrejon formations. In con- 

 trast, in the typical Rocky Mountains of Montana the 

 principal uplift appears to have taken place at the 

 end of Fort Union time — that is, subsequent to basal 

 Eocene time. In the Park Range province of Colorado 

 there was uplift and vigorous erosion at the end of 

 the Cretaceous period and renewed uplift after the 

 deposition of the lower Eocene Wasatch and Green 

 River sediments. 



The separate history of the great mountain ranges 

 in the basin region also shows that the upward move- 

 ments began early in Eocene time. The Big Horn 

 Range of northern Wyoming (Darton, 1906.1) arose 

 as an anticline from the nearly horizontal strata of 

 the Plains to a height of 9,000 feet in early Eocene 

 time. Its uplifted peaks were truncated, and the larger 

 features of the present topography were outlined. 

 The major uplift of the Wind River Mountains, which 

 produced a broad, low, somewhat broken anticline, 

 also took place in early Eocene time (Fisher, 1906.1). 

 In the Wasatch Range of western Wyoming, an east- 

 ward-dipping monocline cut off along its western side 

 by a great fault, there was only a slight uplift at the 

 end of the Jurassic, the main uplift taking place at the 

 end of the Cretaceous (Boutwell, 1907.1). Subse- 

 quent movement took place in post-Eocene time. 

 East of the Wasatch Range is the exceptional east and 

 west anticline of the Uinta Mountains, which extends 

 eastward and westward as a broad central plateau, 

 150 miles long and 30 miles wide, forming a dividing 

 line between the Bridger and Uinta Basins. The for- 

 mation of the Uinta arch began at the end of the 

 Cretaceous period (Emmons, 1907.1, p. 302), as is 

 shown by the fact that the flanking Tertiary beds lie 

 unconformably over the upturned edges of the older 

 strata, which stand at angles of 30° or more. The Eo- 

 cene formations — the Wasatch, Green River, Bridger, 

 and Uinta — are upturned against the flanks of the 

 Uinta Mountains, in a position which means that the 

 continued rise of the mountain mass has dragged up 

 the edges of the adjoining beds. 



Powell estimated that the summit of the Uinta 

 anticline rose 25,000 feet above the level of the ad- 

 jacent country — the Bridger and Uinta Basins. This 

 altitude is equivalent to that of the Himalaya Moun- 

 tains. Certainly in Eocene time the Uinta was a 

 lofty, majestic range. The Colorado Front Range arose 

 between the time of the deposition of the Laramie and 

 Arapahoe formations, to the south, and the San Juan 

 Mountains arose at the end of Cretaceous time and 

 again after the deposition of the basal Eocene Puerco 

 and Torrejon formations. 



