

THE CRETACEOUS PERIOD 747 



The Dakota formation. The Dakota formation, mainly of non- 

 marine origin, is wide-spread in the Great Plains, though buried 

 over the greater part of the area. It extends westward beyond 

 the Rocky Mountains, though in the mountain region the area of 

 deposition was interrupted by elevations which rose above the lakes, 

 marshes, or river flats where the sedimentation took place. The 

 formation is largely sandstone, though it contains much conglom- 

 erate and clay, and some lignite. 



The formation was formerly regarded as lacustrine, but it is 

 perhaps to be regarded rather as the joint product of subaerial and 

 fluviatile deposition. The presence of bird tracks in Kansas/ and 

 the wide-spread abundance of fossil leaves of angiosperms, in a con- 

 dition which precludes much transportation, imply subaerial sedi- 

 mentation to a notable extent at least. The upper part of the for- 

 mation carries some marine fossils. North of Texas the formation 

 is in apparent conformity with the Comanchean in some places, 

 though in others, as in the Wasatch and Uinta mountains, it rests 

 on older formations. 



The formation is an important source of water in the semi-arid 

 plains. It takes in the water where it outcrops near the mountains 

 and the water follows the beds down their dip to the eastward. 

 Along the east base of the Rocky Mountains, where the beds have 

 been tilted, the less resistant formations associated with this sand- 

 stone have been removed or worn down, leaving the outcropping 

 edges of this formation as ridges or " hogbacks" (Fig. 120), char- 

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

 from New Mexico to Canada. 



The Colorado series. 2 The Colorado series records an extensive 

 invasion of the western interior by the sea, the invasion going so 

 far, probably, as to establish a connection between the Gulf of Mex- 

 ico on the south and the Arctic Ocean on the north, over the site 

 of the Great Plains. The western border of this sea appears to 

 have been in Arizona, Utah, eastern Idaho, and western Montana, 

 and the eastern as far east as Minnesota, Iowa, and Kansas. Clastic 



1 Williston, Univ. of Kans. Geol. Surv., Vol. IV, p. 50. 



2 For subdivisions of this series, see Logan, Jour. Geol., Vol. VII, pp. 83-91, 

 and folios of the U. S. Geol. Surv. 



