NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 5 



tionably a large amount of limestone of marine origin in the rocks 

 of Pennsylvanian age in the Southern Rocky Mountain Province, 

 but there are also thin beds of coal and plant-bearing sedimentary 

 rocks which indicate lowlands and coastal swamps in Pennsylvanian 

 time. These have been found in New Mexico, near Socorro; in 

 the mountains east of Albuquerque; near Santa Fe on the western 

 slope of the main range ; in the Pecos Valley between the mountain 

 ranges; east of the main ranges near Las Vegas; and farther to the 

 north in Moreno Valley. Thin beds of coal of Pennsylvanian age 

 have been reported from many places in central and western Colorado 

 and in eastern Utah, both north and south of the Uinta Range. These 

 coal beds are not thick enough to be of commercial value, but they 

 prove that the physiographic conditions of the Rocky Mountain 

 region during the early part of the Pennsylvanian epoch were not 

 so different from those in eastern and central North America as 

 many geologists have supposed. However, later in the epoch these 

 coals were covered by the sea in which was formed the massive 

 limestone of Pennsylvanian age in New Mexico and Southern Colo- 

 rado, which seems to indicate clear water and open sea conditions. I 

 call attention to these facts because it was in the midst of this sea 

 that the ancient Rocky Mountains were elevated. This is particu- 

 larly significant, for we shall see later that this process was repeated 

 when the site of the mountains was covered by the sea in the Upper 

 Cretaceous epoch; and in the midst of this sea were formed the 

 present Southern Rocky Mountains. 



RISE OF ANCESTRAL ROCKY MOUNTAINS 



Some time late in the Carboniferous period these coal-bearing 

 rocks and the marine limestone of Pennsylvanian age were upturned 

 and there followed a period during which the elevated lands furnished 

 red sediment to the neighboring lowlands and seas. Some of these 

 " Red Beds " belong in the Triassic system; others are certainly of 

 Permian age ; and still others, such as the Manzano group, have 

 been classed as Pennsylvanian * on the basis of the fossil inverte- 

 brates, although there is a growing tendency to regard them as 

 Permian. The fossil plants and vertebrates recently discovered in 

 some of the older " Red Beds " tend to establish their Permian age. 

 The subdivision and classification of the " Red Beds " present prob- 

 lems which are not likely to be solved for a long time to come. 



1 Lee, W. T., and Girty, G. H., U. S. Geol. Survey Bull., 389, 1909. 



