NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 29 



early or middle Jurassic time, while the areas farther east were still 

 undergoing erosion, and that the deposits spread eastward as time 

 went on until this upland deposition was interrupted by the invasion 

 of the sea. There is reason, therefore, for correlating the Wingate 

 sandstone with both the White Cliff and Vermilion Cliff sandstones. 

 There is equally good reason for believing that the unconformity at 

 the base of Vermilion Cliff, the only obvious unconformity between 

 the rocks of undoubted Triassic age and the marine Jurassic in Utah, 

 is the westward extension of the great unconformity now well known 

 at the base of the La Plata group in Arizona, New Mexico, and 

 Colorado. 



If the correlations as outlined are correct, it seems probable that 

 most of the material constituting the La Plata group was derived 

 from the highlands in western Arizona, Nevada, and neighboring 

 regions. The thinning of the La Plata where it overlaps onto the 

 Archean in the Rocky Mountain region in central Colorado seems to 

 prove that such lands as existed there at that time were so low that 

 they furnished little sediment. It follows that the mountains in 

 Colorado which had furnished the great quantities of coarse material 

 for the older " Red Beds " had been reduced to a peneplain before 

 La Plata time. 



The old valley in which the sediments of the La Plata group accu- 

 mulated was partly filled with sand and depressed so that the sur- 

 face was near sea-level in late Jurassic time and may or may not have 

 been occupied by a trunk stream. It lay so low that a slight rise of 

 water level caused the marine waters to spread out in it as a broad, 

 shallow sea. It is not known how far this ancient valley-plain 

 extended southward and eastward, but the great length of time that 

 the western interior had been subjected to erosion was sufficient for 

 the reduction to a low-lying plain of any mountains which may have 

 existed. 



I picture the physiographic conditions at the opening of La Plata 

 time something as follows : The broad valley had developed during 

 the early part of the Jurassic period between the ancient mountains 

 of Colorado and the newly formed continental land mass farther 

 west, somewhat similar to the Mississippi Valley of the present day. 

 It may have been formed partly by subsidence, but it seems probable 

 that it was an area cut off from the ocean by uplift to the west and 

 later shaped by erosion. In few places unusual thicknesses of the 

 sedimentary rocks seem to denote local basins caused by down- 

 warping. But the uniform thickness of the sediments which were 



