454 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 30 



the coarser material soon settles, while the finer clay remains sus- 

 pended and falls down very slowly in late fall or winter, when Lake 

 Louise is frozen all over with an ice cover about 40 inches thick. 

 The deposits at the lake bottom are, therefore, found to consist of 

 coarse laj^ers of silt and sand alternating with fine layers of clay; 

 the coarse sediments being laid down during the melting period, the 

 fine clays during the winter when there is no, or else very little, 

 melting of the glacier. There is also a difference in the color of 

 these two layers, the winter layer being, as a rule, of a darker hue.* 



The same conditions as observed at present in Lake Louise occurred 

 very frequently during the melting of the enormous ice sheets which 

 covered the northern part of Europe and North America during the 

 glacial period. At many places the drainage of a district was blocked 

 by the deposits made by the melting ice sheet, when the front of this 

 ice sheet stood for a considerable time at the same spot. As a con- 

 sequence when the ice front later receded the dammed-up melting 

 waters gathered in lakes, forming coarse layers in summer and finer 

 and darker ones in late fall and winter, thus giving origin to banded 

 clays. One pair of these bands (or varves, after the Swedish word) 

 forms in one year, just as an annual growth ring in trees forms 

 during one year. (PI. 1.) 



Such " varved " clays occur on a large scale in southern and central 

 Sweden, where the rivers of the late glacial ice sheets, carrying the 

 detritus of the ice, poured their water into such lakes. Here De Geer 

 began his studies and counted with his students the varves from the 

 Scania Peninsula in southern Sweden to Jamtland in central Sweden, 

 over a distance of 800 kms. (500 miles). He found 5,000 pairs 

 of layers requiring 5,000 years for their formation. Thus the retreat 

 of the ice front from Scania to central Sweden occupied 5,000 years. 

 This retreat was accomplished slowly in Scania, about 75 m. (250 

 feet) a year; farther north it increased to 100 m. (328 feet). Later, 

 the melting set in rapidly and the yearly recession was from 100 to 

 300 m. (328 to almost 1,000 feet). 



At the end of these 5,000 years the shrinking ice was bisected and 

 retreated into the Scandinavian mountains. That was the end of the 

 glacial period and the beginning of postglacial conditions. The dura- 

 tion of postglacial time was in a similar way determined by counting 

 the varves laid down in the postglacial lake Ragunda and in the 

 former fjord of Angermanalven. The result was about 8,700 years 

 with an uncertainty of 100 to 200 years; i. e., postglacial time lasted 

 8,500 to 8,700 years from its beginning until 1900 A. D. Therefore, 

 southern Scania was freed from ice 8,700 + 5,000 = 13,700 years, or, 

 using the lower figure, at least 13,500 years before 1900 A. D. (In 



* W. A. Johnson, Sedimentation in Lake Louise, Alberta, Canada, Amer. Journ. Sci., 5th 

 aeries, vol. 4 (1922), p. 37G ss. 



