QUATERNARY 665 



hickories. In the upper portion (Scarborough beds) the fossil flora 

 indicates a colder climate, showing that the ice sheet was again ad- 

 vancing. 



A second occurrence of interglacial deposit (Aftonian) is in Iowa and 

 yields the bones of a number of animals : elephants and mastodons, 

 giant beavers (Castoroides), camels, horses, some dwarf and some 

 perhaps larger than the domestic horse, and all of extinct species, 

 giant sloths (Megalonyx), etc. This deposit is of especial importance 

 since it gives some idea of the life of the first interglacial period and 

 furnishes a clue to the age of the fossil deposits south of the limit 

 of the ice sheet, since, if the fauna in any of these are the same as 

 that of the above, it is probable that they lived at the same time. 



North and South Migrations during Glacial and Interglacial 

 Times. — We have seen that warm temperate plants, such as the 

 papaw and Judas tree, lived in southern Canada during at least one 

 interglacial period. The discovery of a fossil tamarack (larch) 

 in Georgia, 480 miles below its present limit, shows that the Pleisto- 

 cene climate of the southern states was colder at certain times than 

 at present, and for a period sufficiently long to permit trees to grow. 

 Walruses of a northern type lived on the coast of Georgia, and caribou 

 and moose ranged into Pennsylvania and Ohio. 



An interesting suggestion explaining the present migratory habit of birds is that 

 it is due to the reduction in the temperature of the Arctic regions during late Tertiary 

 and Pleistocene times. During the earlier Tertiary the comparatively uniform climate 

 of the world would not necessitate any extended periodic movements, but the cold of 

 the Glacial Period must have enforced prolonged migration, and periodic migration 

 developed later. "During the waning ice period the areas offering a congenial home 

 to a great multitude of birds became greatly extended, from which, however, they 

 were driven by semi-arctic winters to seek favorable winter haunts farther southward." 



Deposits beyond the Ice Sheets or Protected from Them. — (1) Cav- 

 erns have been the greatest source of our knowledge of the mammalian 

 life of the Pleistocene, the bones found in them having been brought 

 there by floods or by beasts of prey which used the caverns as their 

 lairs. The term " cavern " is used in the broad sense to include true 

 caves, sink holes, and fissures. A section of the cave of Gailenreuth 

 (Fig. 575), showing the bones embedded in clay and the whole covered 

 by a stalagmitic crust, is typical of many European bone caverns. 

 Sometimes, however, there is more than one stalagmitic crust. The 

 number of individuals and species represented by the bones found 

 in some of these caves is remarkable. In the Gailenreuth cave the 



