THE CRETACEOUS PERIOD. 145 



North of the United States, it appears to be represented by con- 

 glomerate overlying the Kootenay series, 1 and beds correlated with 

 it occur in the Frazer River valley farther west. 2000 feet of volcanic 

 material, referred to this epoch, occurs in Crow's Nest pass. 2 



In the Plains region, the Dakota formation is largely sandstone 

 (or quartzite) though it contains much conglomerate and clay, and 

 some lignite. In general, it is coarser to the west and finer to the east, 

 implying more vigorous drainage from the western side. 



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

 been tilted, the less resistant beds associated with the Dakota sand- 



Fig. 391. — Section showing the Cretaceous resting on the Archean. 

 Walsenburg, Colo., folio, U. S. Geol. Surv. 



stone have been removed, leaving its outcropping edges as ridges or 

 " hog backs " (Fig. 392). These ridges are characteristic of the western 

 margin of the Great plains, much of the way from New Mexico to 

 Canada. Locally, the sandstone has a pronounced concretionary struc- 

 ture (Figs. 393 and 394). 



The Dakota sandstone is often an important source of water in 

 the semi-arid plains. It gets its water where it outcrops near the 

 mountains, and the water flows eastward down the dip of the beds. 

 In Dakota and elsewhere many of the deep wells go down to it for 

 water for irrigation and other purposes. 



West of the mountains of Colorado, the area of which was above 

 water, the formation is less commonly sandstone. Clay or shale is 

 here more abundant, and beds of coal of workable thickness give some 

 clue to the physical conditions which prevailed, at least locally. 



The Dakota formation has commonly been regarded as a lacustrine 

 formation, deposited during an epoch of crustal oscillation during which 

 the depth of the basin increased. The necessity for postulating numer- 

 ous oscillations and nice adjustments is largely removed, if the forma- 

 tion be regarded as the joint product of subaerial and fluviatile depo- 



1 Dawson, Bull. Geol. Soc. Am., Vol. XII, p. 77. 



2 Ibid., p. 78. 



