abstracts: geology 607 



that the Morrison is structurally much more closely related to the over- 

 lying Cretaceous than to the underlying formations and that its sedi- 

 ments were deposited on a graded plain formed mainly by degradation, 

 but in a few places by aggradation. The formation lies with apparent 

 conformity on marine Jurassic, but also overlaps onto a variety of older 

 formations to such an extent as to indicate a long interval of erosion 

 previous to INIorrison time. 



The sequence of events is pictured as follows: The mountains of 

 Carboniferous time had undergone erosion throughout the Triassic 

 period and the early part of the Jurassic, so that the encroaching 

 sea of Upper Jurassic time found a nearly level plain extending 

 over a large part of the Rocky Mountain region. At or near the 

 close of the Jurassic period a slight uplift expelled this sea from the 

 continent. This uplift, evidenced by the withdrawal of the sea, 

 seems to have been a part of the general post- Jurassic movement 

 which elsewhere is regarded as the close of the Jurassic period. Soon 

 thereafter the interior of North America l^egan to subside and the 

 streams spread out over the graded plain the sediments which con- 

 stitute the Morrison formation. These accumulated in the shallow 

 basin lately occupied by the Jurassic sea, but also spread beyond its 

 borders over the broad, low-lying peneplain which seems to hav3 

 extended with little interruption from New Mexico to Montana and 

 from Utah to Kansas. These streams formed the swamps, lagoons 

 and shallow temporary lakes in which lived the huge reptiles of Morri- 

 son time. The remarkable uniformity in character and thickness of 

 the sediments is believed to be due to the slow subsidence of a large 

 interior area, a movement that culminated in the formation of the 

 interior basin of the Upper Cretaceous epoch. Lat^r this movement 

 carried the swamp deposits beneath the level of the sea in which were 

 deposited sediments of late Lower Cretaceous age. According to the 

 writer's view, although the character of the sedimentation changes 

 abruptly from the stream deposits of the Morrison to the marine de- 

 posits of the Lower Cretaceous, no long period of time intervened 

 between them, and it follows that the Morrison is of Lower Cretaceous 

 age. 



The conclusion is reached that the physical character of the Morri- 

 son and its relation to contiguous formations indicate that it was de- 

 posited on a peneplain soon after the beginning of the Cretaceous 

 subsidence, when the surface was too near sea level for further degra- 

 dation, but not yet low enough for marine submergence. It is there- 

 fore the first sedimentary expression in the Rocky Mountain region 



