5 i6 



HISTORICAL GEOLOGY 



as are recorded in the unconformities between the Lower and Upper 

 Cretaceous strata and in the difference in their distributions. 



Upper Cretaceous (Cretaceous) 



The Upper Cretaceous was a period of great subsidence (Fig. 486), 

 no other in the earth's history since the Paleozoic being comparable 

 to it. Not only were portions of the Atlantic and Pacific coasts sub- 

 merged, but a vast 

 inland sea covered 

 for a time the cen- 

 tral portion of North 

 America, from the 

 Gulf of Mexico to the 

 Arctic Ocean (Fig. 

 486), separating it 

 into two land masses. 

 Atlantic and Gulf 

 Coasts. — The sea 

 spread over the 

 coastal plains of the 

 Atlantic and Gulf 

 states, and strata 

 composed of sands, 

 clays, chalk, and 

 "green sands" (glau- 

 conite) were accumu- 

 lated. The map (Fig. 

 486) shows the sup- 

 posed distribution 

 better than a written 

 description. At this 

 time the eastern half 

 of the continent was 

 probably a compara- 

 tively flat plain (Kit- 



Fig. 486. — Map showing the probable outline of North 

 America during a portion of the Upper Cretaceous. The 

 inland or epicontinental seas were widespread. Conti- 

 nental deposits are shown in solid black. (Modified after 

 Schuchert.) 



tatinny peneplain) to which it had been reduced during the long ages 

 of the earlier Mesozoic, notwithstanding occasional warpings. 

 Across this plain the Potomac, Susquehanna, and Delaware rivers 

 meandered, probably in very much the same courses as to-day. 



