114 



RADIOBIOLOGY 



Tliis radiation curve has been widely 

 used in Europe not only to explain the 

 Pleistocene glaciations but also to date 

 them. Since it appears not to apply to 

 the final and now chronomctrically ac- 

 cessible closing phases of the glaciation 

 one may be forgiven a certain skepti- 

 cism as to the validity of Milanko- 

 vitch's theon' as applied to the earlier 

 and more grandiose climatic events of 

 the Pleistocene. It mav moreover be 

 pointed out that charcoal from the 

 occupation level of the famous painted 

 cave of Lascaux gives radiocarbon dates 

 of about 13,500 b.c; a much older date 

 would be assigned to this site on the 

 basis of the Milankovitch theory. 



After the ice retreated from eastern 

 North America, a tundra flora de- 

 veloped and then gave place to a 

 spruce-fir forest. This forest has left 

 traces in the form of recognizable pol- 

 len grains of Picea and Abies in the 

 postglacial sediments formed in peat 

 bogs and on lake bottoms. Later, over 

 most of the humid United States, the 

 coniferous forest gave place to a hard- 

 wood forest in which the proportions of 

 different species of trees seem to have 

 oscillated in response to minor climatic 

 changes. Wherever the full sequence is 

 developed, a layer of sediment (zone 

 B ) bearing little pollen other than that 

 of pine is intercalated between the 

 layers (zone A) bearing spruce and fir 

 and those (zone C) bearing mainly 

 oak, hemlock, and other members of 

 the hardwood community. Most 

 workers have suspected that the pine 

 period implies a cool dry climate, but 

 there is a growing tendency to question 

 earlier interpretations. Whatever eco- 

 logical and climatic meaning the pine- 

 pollen zone B may ultimately prove to 

 have, it is one of the most conspicuous 

 features of the micropaleobotany of 

 North American bog and lake sedi- 

 ments. It is therefore of considerable 

 interest to find that whatever condi- 

 tions did produce the pine maximum. 



they moved northward behind the re- 

 treating ice, though at a considerable 

 distance, being established at Cran- 

 berry Glades, West Virginia, about 

 7500 B.C.; at North Branford, Connecti- 

 cut, about 7000 b.c; Anoka County, 

 southern Minnesota, about 6000 B.C.; 

 St. Louis Countv, northern Minnesota, 

 about 5000 b.c; Aroostook County, 

 Maine, about 4000 b.c. Comparable 

 dates are given by material from the 

 ecologically equivalent Boreal of Eur- 

 ope. Whatever the causes of the charac- 

 teristic vegetation of zone B and of the 

 Boreal, it is evident that they are re- 

 lated to the process of deglaciation and 

 not to some world-wide climatic oscil- 

 lation produced by planetary or cosmic 

 variables. 



Though the full archaeological dis- 

 cussion of the radiocarbon dates has 

 not yet appeared, it is evident from the 

 papers already published that man en- 

 tered the New World shortly after the 

 Mankato maximum, if not earlier. 

 Charcoal from occupation sites in a 

 supposedly Mankato alluvium in Fron- 

 tier County, Nebraska, proves to be 

 from 9000 to 10,500 years old. Other 

 artifact-bearing lavers of like age are 

 known in certain North American 

 caves; groundsloth dung, from Gypsum 

 Cave, Nevada, proved to be 8500 to 10,- 

 500 years old, while bark sandals from 

 Fort Rock Cave, Oregon, were made 

 9000 years ago. Burnt bone from a cave 

 in Patagonia, 125 miles east of Ultima 

 Esperanza, proved to be 8600 years old, 

 while sloth dung, not however associ- 

 ated with man, from the more famous 

 eave at the last-named place was 10,- 

 800 years old. It is evident, therefore, 

 that human occupation of the New 

 World had begun bv the nineth mil- 

 lenium b.c. and that the whole length 

 of the continent had been traversed by 

 about 6700 b.c. A final footnote to 

 American history may be added. TTie 

 explosion which produced Crater Lake, 

 Oregon, and killed some trees in the 



