INTRODUCTION 



In all history a knowledge of exact time is vital. Chronology 

 is the thread on which events are tied like pearls on a string. 



The history of the earth was a history without absolute 

 time, without dates, until Gerard De Geer, forty years ago, found 

 a way which enabled him and Ragnar Liden to work out a 

 chronology of the last 13,500 years. De Geer's and Liden's 

 time scale has the year as its unit, and is exact. It is based on 

 annual layers in clay and silt, varves, 1 very much resembling 

 annual rings of trees. The clay was deposited in lakes in front 

 of the receding edge of the ice sheet, and in fiords in northern 

 Sweden after the ice had disappeared. Except for the last few 

 hundred years, this time scale comprises the whole time since 

 the land ice uncovered southernmost Sweden. The retreat of the 

 ice from southern Sweden to Ragunda, 270 miles north-northwest 

 of Stockholm, took about 5,000 years; and the time that has 

 elapsed since then is found by Liden to be about 8,500 years. 

 Accordingly, the uncovering of southernmost Sweden began 

 about 13,500 years ago. 



During 1921 the writer, who had come to America as a member 

 of Baron De Geer's party in 1920, began to work out a chron- 

 ology of North America in late glacial time 2 and to that 

 end studied the recession of the last ice sheet in New England 

 and New York State. The present paper is based upon field 

 work carried on during five and a half months and chiefly 



1 "The Swedish word varv, subst. (old spelling: hvarf), means as well a circle as 

 a periodical iteration of layers. An international term for the last sense being 

 wanted, it seems suitable to use the transcription varve, pi. -s., in English and 

 French, while in German it might be written Warw, pi. -e" (De Geer, 1912, p. 253; 

 1912a, p. 458; for publications cited see List of References at the end of the 

 volume) . 



2 Post-glacial time, according to the Swedish chronology, begins the year after 

 the bisection of the shrinking ice sheet at Ragunda in northern Sweden. The term 

 "late glacial" refers to the time occupied by the recession of the last land ice up 

 to the event mentioned. Accordingly, in this paper the time occupied by the ice 

 retreat in New England will be called late glacial. 



