EARLY MESOZOIC PHYSIOGRAPHY OF THE SOUTHERN 



ROCKY MOUNTAINS » 



By WILLIS T. LEE, Ph. D., Geologist 



united states geological survey 



(With 4 Plates) 



INTRODUCTION 



This paper results from an attempt to work out the ancient phys- 

 iographic history of a part of the Rocky Mountain region which 

 in Mesozoic time seems to have constituted a fairly well-defined phys- 

 iographic unit. Although the history of neighboring provinces must 

 be considered in connection with this one, it is in many ways one 

 which may appropriately be considered independently and with 

 which others may later be compared. The area to which attention 

 is especially directed includes the mountains of Colorado, which 

 extend southward into New Mexico and northward into Wyoming. 

 This has been called the Southern Rocky Mountain Province by some 

 geologists and the Park Range Province by others. From these 

 mountains as a center, our study will lead us in all directions for data 

 which help to interpret phenomena observed in this province. 



Stratigraphy has sometimes been called fossil physiography, and 

 a knowledge of ancient physiographic history should be useful in 

 solving some of the difficult stratigraphic problems of this western 

 region. There is a certain uniformity in natural processes which 

 may be relied upon. We may confidently assume that during the 

 Mesozoic era the same laws were in operation that govern the present- 

 day world. Then, as now, highlands were eroded, lowlands were 

 built up by the debris washed onto them, and basins were filled with 

 sediments. It seems clear that physiography might be used to better 

 advantage than it has been used heretofore by the stratigrapher and 

 the historic geologist. I am confident that a study of ancient geog- 

 raphy and of the evolution of land forms will lead to conclusive results 

 in correlation in certain places where other lines of investigation fail. 



1 Published by permission of the Director of the United States Geological 

 Survey. 



Smithsonian Miscellaneous Collections, Vol. 69, No. 4 



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