ENVIRONMENT OF THE TITANOTHEKES 



53 



The entire topography of the mountain-basin region 

 was thus broadly defined at the end of the Cretaceous 

 period and was accented by uplifts during and after 

 Fort Union (Puerco and Torrejon) time; also after 

 Wasatch and Green River time, following which, from 

 the present Canadian border to northern New Mexico, 

 there was a continuous very gradual uplift. In gen- 

 eral this uplift was earlier and more rapid in Colorado 

 and New Mexico — that is, it occurred before the Fort 

 Union epoch — and more retarded in Montana, where 

 it occurred after the Fort Union epoch. In the Huer- 

 fano Basin the upturn of the western edge of the 

 Huerfano beds amounts to 84°, and although this 

 uplift is local it indicates a considerable movement 

 in the Sangre de Cristo Range after Wind River 

 time (W. Granger, letter, 1919). Ransome (1915.1, 

 p. 362) believes that a large part of the Rocky 

 Mountain uplift followed the deposition of the Fort 

 Union formation. 



CONTRAST IN PHYSIOGRAPHIC CONDITIONS EAST AND 

 WEST OF THE ROCKY MOUNTAIN FRONT RANGE 



During and after the deposition of the conformable 

 Cretaceous formations (such as the Fox Hills and the 

 Laramie) the country bordering the Rocky Mountain 

 range on the east presented a marked physiographic 

 contrast to that lying within the Rocky Mountain 

 basins. Sedimentation east and west of the Rockies 

 was not contemporaneous. 



East of the Rockies. — On the east flanks of the Front 

 Range great river flood-plain systems began in the 

 north in Pierre time and extended toward the south 

 after Fox Hills time. Thus on the western borders 

 of the present Great Plains region rivers had long been 

 spreading out sand over their flood plains in Alberta, 

 forming such deposits as the Belly River sandstone in 

 Pierre time and the Edmonton sandstone in Fox Hills 

 time, and extending southward through Montana to 

 deposit the Judith River sandstone in Pierre time, the 

 Laramie formation of Colorado, the "Hell Creek beds" 

 of Montana, the great Lance sandstones of Converse 

 County, Wyoming, and the Denver and Arapahoe 

 formations of Colorado after Fox Hills time. 



The fact that the Lance sandstones were laid down 

 at the end of Cretaceous time ^ is shown by the 

 remains of the horned and carnivorous dinosaurs found 

 in them, especially Triceratops and Tyrannosaurus. 

 At about the same time Triceratops alticornis flour- 

 ished east of the Front Range of Colorado, during the 

 deposition of the Denver formation, wliich overlies 

 unconformably (by erosion and uplift) the Laramie, 

 the topmost formation of the "conformable Cretaceous 

 series." These great flood-plain deposits, correlated 

 both by their dinosaurs and by flora of the older Fort 



' The United States Geological Survey classifies the Lance formation as Ter- 

 tiaryC?), but the author regards it as of Upper Cretaceous age. 



Union type, mark the beginning of the Rocky Moun- 

 tain revolution as it affected the country to the east. 

 At certain localities, notably along Hell Creek, Mont., 

 south of the Missouri, these fans of much disturbed 

 channel sand and gravel are contemporaneous with 

 undisturbed beds that appear to be lithologically 

 exactly like those of the Fort Union; consequently 

 Fort Union sedimentation began in some regions early 

 in post-Laramie time. 



This long period of mountain erosion and sedi- 

 mentation east of the Rockies came to an end either 

 through heavy forestation or high-gradient river ero- 

 sion, which deposited materials farther east. It is a 

 very significant fact that in the region east of the Rocky 

 Mountains, between South Dakota and northern New 

 Mexico, only sparse lower Eocene sediments (Huer- 

 fano A and Cuchara) are known between Fort Union 

 (basal Eocene) and Chadron (lower Oligocene) time, 

 whereas in the region west of the Front Range sedi- 

 mentation continued through the entire Eocene epoch. 



West of the RocTcies. — In the mountain-basin region 

 from southern Montana to New Mexico the condi- 

 tions during Lance time were very different from those 

 that prevailed east of the Rockies. There was ap- 

 parently erosion and rapid transportation rather than 

 deposition. Within the mountain basins — except 

 around Medicine Bow, near Laramie, and around the 

 Agathaumas sylvestris locality, near Black Buttes, 

 Wyo. — relatively few deposits of Lance age {Tricera- 

 tops zone) have thus far been identified by means of 

 fossils. The Evanston formation, above the Adaville 

 formation, in the typical Wasatch section of south- 

 western Wyoming, according to Berry, contains plants 

 of Fort Union and of Wasatch rather than of Denver 

 age. Similarly the oldest Eocene deposits of the San 

 Juan Basin (the Puerco and Torrejon) are comparable 

 with the Fort Union and not with the older Lance 

 formation; they overlie unconformably beds of prob- 

 able Montana age. In brief, few deposits of Lance 

 time (Triceratops zone) have thus far been identified 

 within the mountain-basin region, although they may 

 be found hereafter. At many places the oldest sedi- 

 ments of the mountain basins lie upon the eroded sur- 

 faces of unquestioned Cretaceous and older formations 

 with pronounced unconformity. 



Physiographic conditions again changed, apparently, 

 for after Lance time sedimentation began vigorously 

 in the mountain-basin region and continued through- 

 out the Eocene until it formed deposits having a com- 

 bined thickness of 9,000 to 11,000 feet. (See table on 

 p. 43.) Not until Oligocene time, when the deposi- 

 tion of these mountain-basin beds probably ceased, 

 was great fluviatile and flood-plain sedimentation re- 

 sumed east of the Front Range, forming the lower 

 Oligocene Chadron beds. 



