I,ATER GEOLOGICAL HISTORY OF NORTH AMERICA 25 



lake laid volcanic ashes show that these Colorado sequoias rivalled 

 in size the present big trees of California. Many additional genera 

 testify to the changes that have taken place in the flora of the Rocky 

 Mountain region since Miocene times. 



A few PKocene plants have been preserved in New Jersey and 

 along the Gulf coast in Alabama. These show that the forests in 

 those regions were much like they are today with some survivors 

 from the older days that have since disappeared. 



The Tertiary period was succeeded by the Quaternary period 

 which corresponds roughly with what is called the Pleistocene and 

 which in turn corresponds approximately to the glacial period or 

 Ice Age. The Pleistocene is often divided into: (1) an earher or 

 preglacial stage which shows in general a minghng of warm tem- 

 perate, and cool temperate forms; (2) a middle or glacial stage with 

 alternating interglacial periods in some of which the climate was 

 above the normal modern optimum for the same latitudes, and 

 during which the biological record shows a succession of forest, 

 field, barren ground, steppe, tundra, and arctic types in some more 

 northern or montane regions; with increasing glaciation and the 

 same succession in an inverse order with the retreat of the glaciers ; 

 and finally there is (3). the so-called postglacial, late or Upper 

 Pleistocene stage which merges with the Recent, and during which 

 our modern flora assumed its present distribution. The post- 

 glacial and Recent may be compared with the previous interglacial 

 stages and there are many arguments in favor of considering that 

 we are now living in a fourth interglacial period since the time that 

 has elapsed since the last ice sheet covered northeastern North 

 America is less than the duration of some of the previous interglacial 

 periods. 



Geologically the Pleistocene deposits are typically glacial in the 

 north and in the more elevated mountainous regions, namely, 

 boulder clay or drift, gravels, till, loess, peat bogs; and in the South, 

 lake, river, eolian, swamp, cave and marine terrace deposits. In 

 the North the floral record is still largely hidden in the for the most 

 part unstudied peat bogs. Farther south it is furnished by the 

 river clays and prehistoric cypress swamps. 



