418 Scientific Intelligence. 



Connecticut, north almost to the Canadian border. It took the 

 ice something like 4100 years to melt back this distance. How 

 long ago the American continental ice sheet began its melting 

 away is not yet known. The average recession of the ice appears 

 to have been about one mile in 22 years, though locally the rate 

 varies from '83 to as much as 1100 feet per year. At times 

 there was even a slight re-advance of the ice sheet. 



It took about 5000 years for the ice to melt back from southern- 

 most Sweden north for 480 miles. Since the ice melted away, 

 another 8500 years has elapsed, so that it is about 13,500 years 

 since the continental ice sheet began its recession in southernmost 

 Sweden. 



The term "varve" is a Swedish word, and in geology 

 signifies the annual cycles of sedimentation of glacial clays, 

 beginning in the coarser, lighter colored material of summer 

 deposition and ending in the darker winter deposit of the finest 

 blue muds, having a greasy feel. Varved clays are all laid down 

 in fresh-water lakes in front of ice lobes, and when glacially 

 derived muds are laid down in the sea, it is said that they are 

 not varved but are homogeneous, in that the coarser material is 

 mixed with the finest of muds. In fjords, however, such muds 

 may also be faintly varved. 



The measuring of the varves and the making of the local 

 graphs to show the varying rate of annual deposition is not a 

 difficult matter, but requires patience, since the work is both 

 laborious and time-consuming. The greatest difficulty of the 

 method lies in finding a succession of closely adjacent clay 

 (not sand) exposures and in correlating these by means of the 

 graphs from place to place. The method tends somewhat to 

 underestimate the total time and never to overestimate it. 

 Commonly, there are two easily distinguishable layers to a varve, 

 but often the lighter colored summer portion will be much 

 thicker and more or less banded, while the darker greasy winter 

 layer is usually not banded. The normal varve, or the cyclic 

 material for one year, is usually much less than one inch in thick- 

 ness, but in the vicinity of drainage the summer deposition, when 

 Of sand, may rise to as much as 12 feet. 



The De Geer method of evaluating varves also reveals the 

 elimatic periodicity of the time of deposition, not only as to the 

 short cycles, but the long ones as well. When fossil leaves are 

 abundant, one can also discern in years the rate of floral adapta- 

 tion and migration. 



The memoir is concluded with a most interesting map by 

 Professor Goldthwait, which plots for the area of New York, 

 Pennsylvania, New Jersey, and the New England States the 

 direction of ice flow, boulder trains, terminal and recessional 

 moraines, and the position of the ice-edge for every 100 years, 



