THE CRETACEOUS PERIOD 421 



and Utah on the west, and as far north as the Arctic 

 Ocean. At this time, then, North America was divided 

 into two land masses (Fig. 441) : one on the east, which, 

 being very low, furnished little but fine sediments to the 



FIG. 440. The same region as in Figure 438 near the middle of the 

 Cretaceous period, showing the retreat of the shore and the changed 

 positions of the zones of sediments. 



interior sea ; and one on the west, which was more rugged, 

 although by no means so mountainous as the same region 

 is to-day. 



By the advance of the first zone of marine deposition 

 spoken of above, a thick layer of clay was gradually built 

 up over the Dakota sandstone, and this in turn was followed 

 by the zone of calcareous ooze, which produced the chalk 

 now found in Kansas, Nebraska, and the Dakotas. The 

 chalk points to the existence of a clear, open sea beyond the 

 reach of mud-laden currents. In this chalk are found not 

 only marine shells, but the bones of large swimming reptiles, 

 and even of birds and flying reptiles. In the latter we seem 

 to have evidence that the winged animals of Cretaceous 

 times were accustomed to fly far out over the sea, as gulls 

 and albatrosses do now. 



The interior sea retreats. The duration of the interior 

 sea was evidently long, but before the close of the Cretaceous 

 period changes began which eventually caused its disap- 

 pearance. The sediments which were being constantly 

 swept into it around its borders helped in some degree to 



