Hutcliinson 



Radioactive Carbon 



115 



process, can be dated, from the wood of 

 these trees, as having occurred about 

 4500 B.C. The radiocarbon dates also 

 throw some light on the chronolog}' of 

 the various stages in the evolution of 

 the Great Lakes, for which the curious 

 reader is referred to Flint and Deevey's 

 paper. 



Summarizing the first geological re- 

 sults of Libby's method, it appears that: 



(1) The last glacial substage in 

 the United States, the Mankato, 

 reached its maximum about 9000 

 B.C., very much later than had for- 

 merly been supposed. 



(2) The very large ice sheet over 

 northern continental North Amer- 

 ica must have receded faster than 

 had formerly been thought possible, 

 almost as fast as did the much 

 smaller Scandinavian icecap. 



(3) The warm episodes, the Cary- 

 Mankato interval in North Amer- 

 ica, and the Allerod oscillation in 

 Europe, were essentially contempo- 

 raneous. 



(4) Such information as is avail- 

 able from radiocarbon dating is not 

 consistent with Milankovitch's as- 

 tronomical theory of glaciation. 



(5) The pine zone (B), evidently 

 the equivalent of the European 

 Boreal, intercalated in manv North 

 American pollen profiles between 

 the zone (A) of the spruce-fir forest 

 and that (zone C) of the hardwood 

 or deciduous forest, represents an 

 episode dependent on the retreat of 

 the ice and occurs at a progressively 

 later date in more northerlv lati- 

 tudes. 



(6) Man entered the New World 

 by the ninth millenium B.C. and 

 penetrated to the extreme south of 

 South America within one or two 

 thousand years. 



If these conclusions were all that 

 could be obtained by radiocarbon dat- 

 ing, the elaboration of the method 

 would have been a most remarkable- 

 achievement of lasting importance. 

 Actually such results are only fragmen- 

 tary beginnings. 



QUESTIONS 



What use can be made of radioactive 

 carbon in dating individual objects and 

 in correlating events in widely sepa- 

 rated areas? 



3. What is meant by "the ninth millen- 

 ium B.C." and what significance does 

 this figure have for people of North 

 America? 



2. When was the last glacial period in 4. Explain fully what is meant by radio- 

 the United States? active carbon. 



