170 ANNALS NEW YORK ACADEMY OF SCIENCES 



Mountain area before the deposition of the Morrison formation, or perhaps 

 before the deposition of the Morrison formation of certain regions in the 

 eastern part of its distribution area. Whether this retreat took place im- 

 mediately after the deposition of the beds now constituting the highest 

 members of the Sundance formation, or whether post- Sundance marine 

 beds were deposited and eroded before the deposition of the Morrison, is 

 a rather difficult question to decide. It is probable that there was a time 

 interval betAveen the deposition of the highest Simdance beds and lowest 

 Morrison beds in the eastern area. 



In the southwestern Morrison area the McElmo appears to overlie the 

 La Plata sandstone conformably. In some localities, however, such as 

 near Green Eiver, Utah, and in northeastern Utah, the McElmo or its 

 representative, the continental part of the Flaming Grorge, overlies the 

 marine Jurassic of Sundance age. It is probable that the continental 

 sedimentation which produced the larger part of the La Plata sandstone 

 was interru]3ted by the invasion of the Sundance sea. If any area where 

 the La Plata had been deposited was not covered by this sea, and there 

 appears to be such, and if this area was in connection with the source of 

 material to the westward, it is probable that continental deposition con- 

 tinued without any extensive break from La Plata into lower McElmo 

 time. In areas in Colorado, east of the southern Sundance sea in Utah, 

 the La Plata sedimentation was suspended for the time being; whether 

 erosion of the Colorado area took place at this time is difficult to deter- 

 mine. The Colorado La Plata probably never extended very far to the 

 east at any time. Possibly slight erosion took place over the Colorado 

 area, but not enough to make any sharp erosion contact between the 

 La Plata and McElmo. 



From the distribution of the Sundance beds, it appears probable that 

 the Sundance sea retreated in the direction from which it advanced, ex- 

 posing the southern areas first. From the nature of the contact between 

 the Sundance and Morrison formations in the eastern areas, it appears 

 possible that post-Sundance beds were deposited and eroded before the 

 deposition of the Morrison. Such an interpretation certainly fits the facts 

 known in the case, and there does not appear to be any strong evidence 

 against it. 



When the southern Utah area was laid bare by the retreat of the Sun- 

 dance sea, a broad practically flat plain seems to have been left, and as 

 the sea retreated farther and farther this plain appears to have become 

 larger and larger, until it occupied a considerable tract in the western 

 portion of the United States. Continental sedimentation probably began 

 immediately after the retreat of the sea. In the southwestern areas the 



