324 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 19 31 



intermediate belt in North America, the uncovering was exception- 

 ally slow. In the Danish Islands, which represent the intermediate 

 zone in Europe, the recession was also slow and repeatedly inter- 

 rupted by large oscillations. Central Ontario and the Danish Is- 

 lands may therefore have been uncovered at the same time. In the 

 regions north of these intermediate belts, both in North America 

 and in Europe, the rate of retreat was rapid for a few thousand 

 years. When the ice fronts had retired to the Cochrane region in 

 Ontario and to central Sweden and southern Finland, respectively, 

 they halted and oscillated. Also these stages of recession and halt 

 perhaps correspond, though it is important to note that the oscilla- 

 tions at Cochrane rej)resented a much longer time than did those in 

 Fenno-Scandia which amounted to 659 years. For the present the 

 attempts at trans-Atlantic correlation can hardly be carried beyond 

 these general suggestions. 



Because the rate of retreat of the ice front and the thickness of 

 the clay varves form direct measures of the amount of the relative 

 summer heat, they furnish excellent material for the study of long 

 as well as short temperature cycles. However, few analyses for de- 

 termining cj'cles have been made. The record of the summer heat 

 supplied by the varved glacial clay is the longest known. The clay 

 chronology, even though incomplete, sheds light on several problems 

 not touched upon in the preceding, as, for instance, the rate of ero- 

 sion, rate of development of shore lines, rate of sedimentation, rate 

 of leaching and weathering of soils and rocks, rate of vertical move- 

 ments of the earth's crust resulting from the removal of the ice load, 

 rate of occupancy by plants and animals of the lifeless regions that 

 were exposed with the melting of the ice sheets, rate of develop- 

 ment of plant and animal associations, and on the rate of evolution 

 of human species, races, and cultures. 



