536 



THE CRETACEOUS PERIOD 



the joint product of subaerial and fluviatile deposition. The pres- 

 ence of bird tracks in Kansas, and the widespread abundance of 

 fossil leaves of angiosperms, in a condition which precludes much 

 transportation, imply subaerial sedimentation to a notable extent 

 at least. The upper part of the formation carries some marine 

 fossils. North of Texas the formation is in apparent conformity 

 with the Comanchean in some places, though in others it rests on 

 older formations. 



The Dakota sandstone is an important source of water in the 

 semi-arid plains. The water enters where the sandstone outcrops 

 near the mountains, and follows the beds down their dip to the east- 

 ward. Along the east base of the Rocky Mountains, where the beds 



Fig. 453. A group of concretions weathered out from the Dakota sandstone. 

 Near Minneapolis, Kan. (Schaffner.) 



have been tilted, the less resistant formations associated with this 

 sandstone have been removed or worn down, leaving the outcropping 

 edges of this formation as ridges or "hogbacks" (Fig. 93), char- 

 acteristic of the east base of the Rocky Mountains much of the way 

 from New Mexico to Canada. 



The Colorado series records an extensive invasion of the western 



