8 GEOLOGY OF THE DENVER BASIN. 



150 miles wide on a north-and-south line. The divides between these 

 basins and those on the north and south, respectively, are flat-topped ridges, 

 sloping gently eastward from the foothill region and having average eleva- 

 tions 2,000 to 3,000 feet above the lowest points of the basins. 



The more enduring beds which form the divide south of the Arkansas 

 Basin are the basal sandstones of the Laramie, capped to a considerable 

 extent by still harder sheets of basalt. The Arkansas Basin itself is largely 

 eroded out of the clays of the .Middle Cretaceous. The Arkansas-Platte 

 divide is formed of the conglomerates and coarse sandstones of the Monu- 

 ment ('reek series, in part capped by beds of rhvolitic tuff. Hie South 

 Platte Basin is largely eroded out of the clays of the Upper Laramie and 

 later formations, and the divide to the north, between it and the North 

 Platte, consists of Miocene limestones and conglomerates of Pliocene age. 



Within these basins the details of the topography are dependent on 

 more recent geological phenomena. Thus, in the Denver Basin area, which 

 occupies the southern half of the South Platte Basin, while the broader 

 outlines of its topography were roughed out in Tertiary time, these have 

 been more or less effaced by Pleistocene deposits of river drift and loess, 

 upon the subsequent erosion of which the present details of its topographical 

 form are mostly dependent. 



lielics of the older topography of this region may be distinguished 

 in the mesa-topped spurs of the Arkansas-Platte divide, only the extreme 

 points of which appear within the area of the map, but which are charac- 

 teristically developed in the vicinity of Castle Pock, just south of that 

 area. Green Mountain and the twin mesas known as the Table Moun- 

 tains, near Golden, are other features which have not been essentially 

 changed since the Tertiary erosion. 



In the more modern features are to be recognized remains of a series 

 of terraces, some of which are undoubtedly ancient river terraces of a 

 period of earlier Pleistocene erosion, others being apparently lake terraces, 

 formed in a sheet of water of lake-like dimensions, which occupied the 

 area subsequent to this period. 



The most prominent river terrace is that along the east side of the 

 Platte River. Its surface is uniformly horizontal or with a slight slope 



