NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 3 



and there is seeming conflict of evidence. Different classes of data 

 now seem to lead to contradictory conclusions. Conflict of evidence 

 is only another expression for misinterpretation of evidence. There 

 is no conflict when all facts are known, and I am convinced that 

 physiographic principles can be used to great advantage in cor- 

 relating some of the unfossiliferous sedimentary rocks in the moun- 

 tain region. 



A PRELIMINARY SUMMARY 



The succession of events outlined in this paper begins with a 

 time in the Carboniferous period when the sea covered the region 

 where the Southern Rocky Mountains now stand. This sea was 

 expelled and the ancestors of the present Rocky Mountains arose 

 in its place. For long ages these mountains withstood the elements 

 but were finally torn down and swept away. Before they had entirely 

 disappeared other lands were elevated farther west and on them 

 mountains were thrown up. Probably these new mountains were 

 high, for a desert developed east of them, perhaps for the same 

 reason that desert conditions prevail now east of the high mountains 

 of California. The moisture from the Pacific was precipitated on 

 these mountains and the streams carried the rock waste out into the 

 desert, where the winds reworked it, piling the sand into dunes, 

 which are now recognizable in their fossil state. 



A broad depression or valley somewhat similar to Mississippi 

 Valley, except that it drained northward, developed between the new 

 mountains and the ancestral Rockies. In the western part of this 

 valley the dune sand accumulated to great depths and graded off 

 toward the east, covering the lower parts of the older, deeply eroded 

 mountain area, but leaving the hilly parts uncovered. After the 

 sands had accumulated in eastern Utah and neighboring regions to a 

 depth of nearly 3,000 feet, the sea advanced in late Jurassic time 

 up the old valley, across British Columbia and the Mountain States 

 as far south as northern New Mexico and Arizona. Much of the 

 submerged area was nearly flat and the sea was shallow in most 

 places. Around it, especially in Utah, New Mexico, and Colorado, 

 were shallow, partly inclosed bays where gypsum was precipitated 

 by evaporation of the sea water. Extensive beds of gypsum have 

 been found in many places which mark the location of these ancient 

 evaporation pans. 



The sea was short-lived, and as it retreated, sand drifted over the 

 abandoned areas, covering in some places, but not in all, the gypsum, 



