Hutchinson 



Radioactive Carbon 



113 



vestigators by the Institute of Nuclear 

 Studies of Chicago University, and has 

 been pubhshcd with a few additions in 

 a paper by Arnold and Libby. The 

 geological implications of these dates 

 have been considered at length by Flint 

 and Dee\'e\', and some of the European 

 results have also been discussed by 

 Godwin. It is these contributions which 

 will primarily be considered here. 



So far, much the most important 

 contribution made by Libby and his 

 co-workers to the chronology of the late 

 Pleistocene has been the dating of the 

 Two Creeks Forest Bed of Manitowoc 

 County, Wisconsin. This bed consists 

 of peaty material containing spruce 

 logs, mostly pointing southwest, and 

 with splintered ends. The bed is over- 

 lain by a thin lacustrine deposit and 

 above this by till representing the 

 Mankato substage, the last glacial re- 

 advance recorded within the United 

 States. Evidently, the site was occupied, 

 during the somewhat warmer Cary- 

 Mankato interval, by a spruce forest. 

 This became flooded while the trees 

 were still standing and later the advanc- 

 ing ice toppled the trees over and fin- 

 ally covered them with ground mo- 

 raine. The Mankato ice sheet advanced 

 about twenty-five miles further south 

 and then began its final retreat. Until 

 recently most, though not all, students 

 of Pleistocene glaciation in North 

 America believed that the Mankato ice 

 achieved its maximum southward ex- 

 tension about 25,000 years ago. 



Various pieces of wood, collected in 

 the Two Creeks Bed by L. R. Wilson, 

 J. H. Bretz, and L. Horberg, have pro- 

 vided a most encouragingly concordant 

 set of radiocarbon dates indicating that 

 the spruce trees of the Cary-Mankato 

 forest actually were growing between 

 10,877 and 12,168 years ago. The mean 

 age is 11,404 ± 350 years. Since the 

 Two Creeks forest grew very close to 

 the edge of the Mankato ice sheet, one 

 can safely assume that the time of the 



maximum development of that ice 

 sheet was about 11,000 years ago or 

 about 9000 b.c. 



A warmer episode, comparable to 

 the Cary-Mankato inter\'al, followed by 

 renewed refrigeration, has been recog- 

 nised in Europe as the Allcrod horizon, 

 identified in a number of localities from 

 Ireland eastward into Russia. The Al- 

 lerod was apparently followed bv a 

 glacial readvance, termed the Fen- 

 noscandian in Northern Europe. Four 

 out of five samples of supposedlv Al- 

 lerod material, from Ireland, England, 

 and western Germany, collected bv G. 

 F. Mitchell, H. Godwin, K. B. Black- 

 burn, and Franz Firbas, give concor- 

 dant dates, ranging from 9861 to 11,310 

 years old, this overlapping the narrow 

 range of possible dates for the Two 

 Creeks Bed. It is thus reasonably cer- 

 tain that these two oscillations in cli- 

 mate are contemporaneous and equiva- 

 lent. This is a result of extraordinary 

 interest. It permits for the first time 

 accurate correlation of the closing 

 events of the Pleistocene glacial on 

 either side of the Atlantic. It shows 

 that the disappearance of the verv large 

 ice sheet from continental North Amer- 

 ica took little if any longer than the 

 time of the waning of the Scandina- 

 vian ice, rather than a period more than 

 twice as long, as had often been sup- 

 posed in the past. It also shows that the 

 Allerod oscillation and the subsequent 

 Fennoscandian readvance were not 

 purely local matters but, having their 

 counterparts in the New World, were 

 presumably due to the variation of 

 whatever planetary and cosmic factors 

 may be involved in producing glacial 

 maxima. This is a matter of consider- 

 able interest because local causes 

 have been postulated to explain away 

 the fact that there is no minimum 

 corresponding to the Fennoscandian 

 substage in the "radiation curve" con- 

 structed by Milankovitch from preces- 

 sional and other astronomical changes. 



