386 THE AGE OF MAMMALS 



Geologic deposits. — The chief geologic formations or deposits of glacial 

 times are the following: 



Glacial boulders, boulder clays, and drift. 



Lignitic and swamp deposits, 



Fluviatile gravels, till, and river terraces. 



Lacustrine and marine terraces. 



Loess, fine, calcareous, fluviatile and aeolian loam. 



Volcanic travertines and tufas. 



Phosphorites and other fissure deposits. 



Cave deposits. 



Loess, found in the Pleistocene of Europe, northern Asia, North America, 

 and in the pampean regions of South America, is the mo.st distinctive of all 

 glacial deposits, next to the boulder clay and drift. It consists of a fine, 

 porous, silicious and calcareous silt, usually of a light brown color, charac- 

 terized by a peculiar competency to stand in vertical walls during erosion. 

 Its distribution is quite independent of altitude, occurring in Europe from 

 sea level to a height of 1,500 meters. Its origin is partly fluviatile, partly 

 seolian. Thus it is believed that the fine mud carried by rivers becomes 

 desiccated and is retransported by the wind. Penck (1904) describes loess 

 as formed in districts traversed periodically by great streams, leaving 

 dry mud which is redistributed by the wind. In Europe it is one of the 

 most characteristic formations of the interglacial epochs; some loess de- 

 posits belong to the Riss-Wiirm Interglacial, others are known from the 

 older Mindel-Riss Epoch, and some very rare deposits probably date back 

 to the Giinz-Mindel Interglacial epoch. ^ Another theory of formation is 

 that the snow driven by the wind carried earthy material with it. Thus 

 the loess remained as a residue after the melting of the snow.- 



The plateau between Uzes and Avignon^ contains numerous' fissures 

 filled with phosphorites which can scarcely be distinguished from those of 

 Quercy but contain the remains of mammals of mid-Pleistocene age. 



I. PLEISTOCENE LIFE OF EUROPE 



Flora and Clhnate 



It is clear from the great fluctuations of temperature and moisture which 

 occurred during Pleistocene times that the flora cannot be treated as a 

 unit nor as progressing in a single direction like the flora of the preceding 



^ Penck, A., Die alpinen Eiszeitbildungen und der prahistorische Mensch. Arch. Anthro- 

 poL, U.S., Vol. I, no. 8, 1904. 



2 Davidson, Charles, On Deposits from Snow Drifts with Especial Reference to the 

 Origin of the Loess and the Preservation of Mammoth Remains. Quart. Jour. Geol. Soc. London, 

 1894, p. 472. 



^ Deperet, Sur Ics phosphorites quaternaires de la region de UzSs. C. R. Acad. Sci. Paris, 

 Tome 120, 1895. 



