40 GEOLOGY OF THE DENVEB BASIN. 



That there has been a general differential uplift of the mountain or 

 subsidence <>t" the plains area — a continued action of the same forces which 

 produced the upturning of the Mesozoic (including the Arapahoe and 

 Denver) beds — is indicated by the observed upturning, at angles of from 

 1 5 to 20 , of the Monument < !reek beds near the Hanks of the mountains. 

 Elsewhere, so far as observed, they do not depart from a practically hori- 

 zontal position, and apparently have not been subjected to deformation 

 resulting from a general orographic movement. Movements of elevation 

 and subsidence, rather of an epeirogenic or continental nature, are indicated 

 by both Tertiary and Pleistocene deposits that have a lacustrine origin, 

 since the present inclination of the plains region, which shows an average 

 descent in round numbers of L0 feet to the mile from the foothill region to 

 the valleys of the Missouri and Mississippi, would not admit of the holding 

 of lake waters on its surface. 



It has already been suggested by earlier writers that the present con- 

 ditions of the Tertiary deposits of the plains region indicate a differential 

 tilting of this region which has produced a relative change of level of 

 5,000 feet or more between its eastern and western borders. The area 

 of the present investigation has been too circumscribed to furnish much 

 additional data on this subject. It can only he said that movements of 

 this general nature have in all probability been several times repeated 

 during Tertiary and Pleistocene times, but until the extent and character 

 of these deposits shall have been carefully studied over the whole plains 

 region and their relations to the underlying beds determined it will he 

 impossible to trace with any approach to accuracy the nature and history 

 <>f these movements. 



PLEISTOCENE FORMATIONS. 



In the absence of any phenomena in the region that can definitely he 

 assigned to Pliocene time, whatever has occurred since the deposition and 

 elevation of the Monument Creek heels is provisorily assumed to he post- 

 Pliocene or Pleistocene. In this period the present drainage areas of the 

 plains took definite form. The Monument Creek beds were removed from 

 a part, and possibly from nearly the whole, of the area mapped, and the 

 present outlines of the Platte Basin were generally established. How much 



