420 



HISTORICAL GEOLOGY 



there deposited; it is more 

 probable that they fell di- 

 rectly into the sandy shal- 

 lows of rivers and bayous, 

 and were in turn covered by 

 the sands. Being a porous 

 layer between beds of dense 

 shale, the Dakota sandstone 

 serves as an important reser- 

 voir for underground water, 

 and from it many artesian 

 wells in the northern part 

 of the Great Plains derive 

 their flow. 



Beyond the shore line, in 

 the open sea, very different 

 deposits were being laid 

 down, of course, chiefly 

 clays in a broad belt near 

 the shore, and limy ooze 

 farther out in the clear water 

 (Fig. 440). The shells and 

 bones of marine animals 

 became embedded in these 

 deposits, and are now found 

 as fossils in the Cretaceous 

 rocks. These two zones, in 

 which clay and ooze were 

 deposited respectively, like- 

 wise migrated northward as 

 the sea advanced in that 

 direction, until finally they 

 had overspread the Great 

 Plains and Rocky Mountain 



FIG. 439. - Angiosperm leaves from the reg i on f rom Oklahoma and 

 Dakota sandstone. A, Poplar ; B, Wil- ft . 



low ; c, Sassafras. Iowa on the east to Arizona 



