54 



TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



LATERAL AND MAIN RIVER SYSTEMS IN THE MOUNTAIN- 

 BASIN REGION 



The great mounfain-hasin valley. — The contour lines 

 of the basal Eocene and lower Eocene sediments of 

 the mountain-basin region in northern New Mexico 

 and Montana are very illuminating. They show the 

 presence of a series of broad, relatively level basins — ■ 

 a chain of flat uplands or valleys — in which the prod- 



UINTA BAS 



Period of volcanic 

 ^ dust eruption in 

 ^ soutlnern Wyoming, 

 Q> Utah. and Colorado, 

 sandstones washakie basinQ, mingled with erosion 

 '^ products. 



Chiefly dacite 

 > tuffs and 

 sandstones 



Figure 39. — Chronologic relations of formations in the mountain-basin region 



This diagram exhibits the overlapping of sediments and the falls of volcanic ash in eight widely separated areas (Nos, 

 2-5 and 7-10, flg. 35), which, when combined, cover the entire Eocene epoch. 



ucts of erosion and the volcanic dust that were gath- 

 ered by streams from the surrounding mountains were 

 spread wide, indicating that although the mountain 

 streams had high gradients and great erosive power 

 the larger rivers had low gradients and little trans- 

 porting power. The uniform elevation of the moun- 

 tain-basin region at the north and the south and the 

 low river gradients were favorable to sedimentation. 

 We observe, moreover, that in basal Eocene time the 



conditions of climate and of sedimentation were some- 

 what uniform in the Puerco and Torrejon deposits 

 of the San Juan Basin in New Mexico, laid down by 

 tributaries of Colorado River, and in the typical Fort 

 Union deposits of Montana, laid down by tributaries 

 of Missouri River. The rates of sedimentation were 

 different. Similar basal Eocene sediments probably 

 underlie some of the Wasatch (lower Eocene) deposits 

 in the intermediate basins of 

 Wyoming and Utah, for they 

 have been exposed in the San 

 Juan Basin only by the removal 

 of the overlying Wasatch. A 

 new sedimentary phase was 

 begun in Wasatch time, and a 

 third phase in Bridger time. 



The contrast in the physio- 

 graphic conditions east and 

 west of the Front Range has 

 a very important bearing upon 

 the paleontologic records. The 

 mountain-basin sediments af- 

 ford a marvelous and almost 

 unbroken record of mammalian 

 evolution in the Eocene, but 

 little or nothing in the Oligo- 

 cene, doubtless because large 

 areas of Oligocene sediments 

 have been eroded away. Only 

 two spots remain — Bates Hole 

 and Beaver Divide, in Wyo- 

 ming. 



Piedmont, flood-plain, and la- 

 custrine deposits. — King led the 

 earlier geologists in presenting 

 the theory that the mountain 

 basins were once filled with a 

 chain of lakes. This theory was 

 adopted by Marsh, Cope, Scott, 

 and Osborn. Leidy, as early as 

 1869, cast doubt upon the lake 

 theory as applied to the White 

 River group east of the moun- 

 tains. The lake theory has grad- 

 ually been replaced by the flood- 

 plain theory through the studies 

 of Haworth (1897. 1), Gilbert 

 (1896.1), Matthew (1899.2), 

 Davis (1900.1), Johnson (1901.1), and Hatcher (1902.3). 

 For the highly diversified mountain-basin region 

 throughout the very long period of the Eocene, with 

 its considerable climatic vicissitudes, no single theory of 

 deposition is adequate. We have seen that in the 

 basal Eocene, during Fort Union, Puerco, and Torrejon 

 (Thanetian) time, there were doubtless great level 

 areas, heavily forested, with dense undergrowth, favor- 

 able to the formation of peat and lignitic deposits 



Period of mountain 

 erosion of granitic, 

 calcareous, rhyolitic 

 early volcanic and 

 sedimentary areas 



