236 ROLLIN T. CHAMBERLIN 



which were very close to sea-level. The Colorado sea-level at 

 the close of the Cretaceous thus becomes the critical level. It 

 was a plain approximately at sea-level which was arched and 

 wrinkled into the Rocky Mountains by the diastrophic throes of 

 the dying Mesozoic. Erosion by the rapid streams which sprang 

 into being assailed the uplifted ranges and in time reduced them to 

 the condition of a peneplain. But was, or was not, the level of 

 this peneplain approximately the same as the level of the plain 

 which earlier was folded into the mountains, and whose p©sition. 

 today is such an important factor in our problem ? 



If it were known that the peneplain now visible in various por- 

 tions of the Front Range had been developed at an elevation not 

 greatly above sea-level, and that no changes of level had occurred iii 

 the interim between the period of folding and the development of 

 this peneplain, then the latter could be accepted with confidence as 

 the datum-plane, and it would be proper to measure the height of 

 the projected folds above the present height of the peneplain. 

 This of course presupposes that the orogenic folding movements 

 were accompanied by little epeirogenic uplift. If, however, there 

 was general regional uplift along with the folding this can be treated 

 as subsequently indicated. But there were, in all probability, 

 some changes of level during this very considerable period of time 

 between the folding and the peneplain. A rise of the sea consequent 

 upon the transfer of sediment from the uplifted lands to the ocean 

 floor might confidently be expected. But at most this would 

 amount only to a few hundred feet.^ Furthermore it is to be 

 noted that such a rise of the sea-level is in the nature of a return to 

 the late Cretaceous conditions of widely transgressive seas, and 

 hence toward the conditions and level from which the Rockies 

 arose. 



Far more to be feared are possible uplifts due to vertical 

 forces operating independently of folding movements. There 

 are two possibilities: A general regional uplift might have 

 affected the entire area more or less uniformly; or the change in 

 level might have been confined to local upbo wings, either within 



' R. D. George has estimated that the cutting away of the present continents and 

 the deposition of the material in the ocean basins, would raise the sea-level about 

 650 feet. Chamberlin and Salisbury, Geology, I, 545. 



