294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



there during the advance and retreat of the ice. In nonglaciated 

 regions it is difficult to find traces of changes of temperature; but 

 changes in rainfall leave clear records in river deposits and in the 

 levels of lakes with small or no outlets. From such evidence there is 

 no doubt that there were long and pronounced periods of heavy 

 rainfall during the Pleistocene Period in practically all parts of the 

 world. 



In North America the Great Basin, which is now an arid, sandy 

 desert, was full of water, and the small existing intensely salt lakes 

 were vast sheets of water to which the names Lake Bonneville and 

 Lake Labontan have been given. According to Gilbert and Russell, 

 these lakes were twice filled during the Pleistocene Period, thus 

 indicating that in North America there were two pluvial periods. 



Wayland has reached the conclusion that in Uganda there have been 

 two pluvial periods, one near the beginning of the Pleistocene Period 

 and one toward the end; between these two pluvial periods the cli- 

 mate was much drier than it is today. Huzayyin has also recognized 

 two major pluvials during the Pleistocene in southwest Arabia. 2 



Pluvial periods were not confined to tropical and subtropical regions, 

 for in the Swanscombe Gravels we have evidence of a much greater 

 Thames than exists today. The Corbicula fluminaris contained in 

 these gravels show that they were laid down during an interglacial 

 epoch when the temperature was considerably higher than at present. 



Practically all deserts such as the Sahara, Libyan, and Kalahara 

 show signs of water action in relatively recent times, and many of 

 them are littered with the flint tools of prehistoric man, showing that 

 in the latter half of the Pleistocene Period they supported sufficient 

 animal life to make them good hunting grounds. 



It is probably too early to say definitely that in all parts of the world 

 there were two pluvial periods, but the evidence is very strong, es- 

 pecially in North America and Africa. 



The climatic conditions during the Pleistocene Ice Age may be 

 summed up as follows: 



The Pleistocene Ice Age did not consist of a simple advance and 

 retreat of the ice but of a sequence of glacial and interglacial epochs. 

 Based on observation in the Alps, there were four glacial epochs 

 separated by three interglacial epochs. During some at least of the 

 interglacial epochs the climate was warmer and wetter than today. 

 In regions not subjected to glaciation there is evidence of two great 

 pluvial periods, one early and one late in the Pleistocene Period, 

 separated by a long interval during which there was even less pre- 

 cipitation than at present. 



> Nature, vol. 140, p. 513, 1937. 



