CHAPTER XXIV 

 THE TERTIARY PERIOD 



Results of the warping and folding. The crustal dis- 

 turbances which brought the Mesozoic era to a close wrought 

 great changes in the land forms on the continent of North 

 America. In the Appalachian region a broad swelling or 

 upwarp of the Cretaceous peneplain 1 had raised its surface 

 some two or three thousand feet, and the streams, thus reju- 

 venated, were already engaged in etching out the softer 

 strata, leaving the harder ones protruding as mountain ridges. 



The great central portion of the country had been raised 

 very little, but in the Cordilleran region of the West, the 

 comparatively low-lying Mesozoic surface had been con- 

 verted into mountains of considerable height with interven- 

 ing basins and valleys. There, as in the Appalachians, the 

 hills and mountains were being worn down and the resultant 

 sediments were filling up the lowlands. 



Divisions of the period. The Tertiary period, while per- 

 haps no longer than many that preceded it, is of course much 

 better known, because it is nearer the present. It is usually 

 divided into several epochs 2 : 



(3) Pliocene (more recent). 



(2) Miocene (less recent). 



(1) Eocene (dawn of the recent). 



Additions to the Atlantic and Gulf coastal plain. From 

 New England south to Florida, and almost encircling the 

 Gulf of Mexico, the Tertiary sediments are found lying upon 

 the Comanchean and Cretaceous deposits which had formed 



1 See page 414. 



2 Of these, the Eocene is probably much longer than either of the others, 

 and is often divided into Eocene (proper) and Oligocene. 



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