562 EOCENE AND OLIGOCENE PERIODS 



great erosion which followed the post-Cretaceous deformation. On 

 the physical side, therefore, there is no warrant for assuming that 

 the period was^ short. The faunal developments of the period were 

 such as to make great demands upon time, and it is not improbable 

 that the period was as long as the average of those of the Paleozoic 

 and Mesozoic eras. 



Such thicknesses of terrestrial sediment a<s occur in the Eocene 

 of western North America, if they are really as great as reported, 

 call for explanation. If the areas concerned were in process of 

 more or less continuous warping, low areas going down as surround- 

 ing lands went up, or if troughs or basins of deposition were pro- 

 duced by faulting, the bottoms sinking while their surroundings rose, 

 the conditions would perhaps be met. 



The relations of the Eocene beds of the western interior indicate 

 that both the attitude and altitude of the surfaces in that part of 

 the continent were very different from those which now exist. That 

 region must have been much lower than now, and, locally and tem- 

 porarily at least, without well-established drainage. The present 

 mountains were certainly not so high as now, though considerable 

 elevations and great relief doubtless existed. 



Close of the Period in North America 



The closing stages of the Eocene were marked by crustal move- 

 ments in the west, resulting in considerable changes in geography. 

 Some such movements had taken place during the period, as has 

 been indicated; but the faulting and folding at its close were on a 

 larger scale. The result was the retreat of the sea along the Pacific 

 coast, the development of new areas of high and low lands, and there- 

 fore a shifting of the sites of rapid degradation and aggradation. 



Along the Atlantic and Gulf coasts the Miocene is in many 

 places unconformable on the Eocene, and it was at the close of the 

 Eocene (or perhaps during the Oligocene) that an island, now in- 

 cluded in the peninsula of Florida, was formed. In the Carolinas, 

 and in the western Gulf region, the conformity between the Eocene 

 and Oligocene formations seems to preclude notable changes of 

 geography along the coast in the southeastern part of the United 

 States at the close of the Eocene. 



Foreign 



Europe. Considerable lakes, estuaries, and perhaps other 

 areas of deposition remained over western Europe, at the close of 



