4 GEOLOGY OF THE DENVER BASIN. 



At various periods during Paleozoic and Mesozoic time the South 

 Park depression was connected directly with the ocean, probably through 

 the Canyon City Bay. 'This connection was broken during the period of 

 orographic movement and volcanic activity that followed the deposition of 

 the Laramie coal measures at the close of the Cretaceous. Subsequent to 

 this period, but at how late a date in the Tertiary it is not yet possible to 

 determine, a new drainage channel tor this interior valley was started across 

 the mountains in a general northeasterly direction, which has since devel- 

 oped into the canyon gorges of the South Platte River. Here, as in the 

 Arkansas Valley, a considerable portion of the gorge cutting, especially in 

 the lower part of the valley, is demonstrably more recent than the Glacial 

 epoch, or the period of greatest extent of ice covering over the high 

 mountains. 



The Middle Park now drains westward through the Grand River to 

 the Pacific, lint was during Mesozoic time a nearly continuous depres- 

 sion with the adjoining valleys of South and North parks. The latter is 

 drained by the north fork of the Platte River, which circles round the 

 northern end of the Colorado mountain-system. Hence, neither forms any 

 essential part of the region under consideration. Of the date of this stream 

 it can only he said that its gorge cutting is distinctly later than beds which 

 have been assumed to be of Pliocene age. 



The mountain slopes immediately facing the plains appear, to an 

 observer regarding them from a distance, to rise abruptly out of the sea, as 

 it were, in one continuous but rugged slope. A closer examination shows 

 that while the spurs immediately adjoining the shore-line are abrupt, and 

 the intervening gorges deep and narrow, these spurs, after reaching a certain 

 height, slope hack more gradually toward the main crest, which lies unex- 

 pectedly far back; that the mountain forms are generally smooth and 

 rounded, and that the valleys become wider and more open with increasing 

 elevation- PI. VI, a photographic view looking westward from Coal ('reek 

 Peak across the valley of Upper Coal Creek, shows the region intermediate 

 between the foothill slopes and the crest of the range, with its characteristic 

 rounded slopes and open valleys. The topographical form of the interior 

 region above a given elevation suggests that it has been exposed to secular 



