CHAPTER 



10 



EVOLUTION AS SEEN IN THE 



GEOLOGIC RECORD: 



CENOZOIC ERA 



We have seen that the end of the Mesozoic was marked 

 by momentous changes in the surface of the earth, including the elevation 

 of our Rocky Mountains. As a result the continent of North America at- 

 tained nearly the outlines and general appearance it has today. Geologic 

 changes during this era were largely connected with wearing down of 

 mountains, followed by their reelevation through regional uplift. These 

 phenomena were particularly characteristic of western North America. 

 Then in the Pleistocene period the face of the northern portions of the 

 continent was altered by the action of great glaciers, sweeping southward 

 from arctic regions. 



The Cenozoic era is divided into a succession of periods: Paleocene, Eo- 

 cene, Oligocene, Miocene, Pliocene, Pleistocene, Recent (Table 7.1, p. 

 137). These subdivisions of the time scale are frequently termed "epochs," 

 since they are of a somewhat different order of magnitude from the pe- 

 riods into which the Paleozoic and Mesozoic eras are divided. For the sake 

 of simplicity, however, we shall refer to these Cenozoic divisions also as 

 periods. They are represented by series of strata occurring in the order 

 given and marked by the progressive evolution of animals, particularly of 

 mammals. Indeed, the Cenozoic is sometimes called "the age of mam- 

 mals." 



The periods were of unequal duration. As noted earlier (p. 138), the 

 Cenozoic is estimated to have lasted from about 63 million years ago until 



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