4 ICE RECESSION IN NEW EXGLAXD 



in 1894 (p. 2S8). All of them used this peculiarity as a means 

 for determining the time occupied by the formation of certain 

 clay deposits, but De Geer (1884, 1885) alone went farther and 

 found a way to make it the basis for a chronology of the closing 

 stage of the Ice Age. His method is the following. 



Since the varves were deposited in front of the receding ice 

 edge, they cover each other like shingles on a roof (Fig. 1). 

 Since, also, their thickness varied from year to year and stood 

 in direct relation to the amount of annual melting, which in 

 its turn was chiefly determined by the summer temperature 

 (see p. 85), it is possible to recognize the varves from widely 

 separated localities, provided they lie within a climatologically 

 uniform area. Furthermore, it is possible to determine the 

 rate of the retreat of the ice. To this end exposures in clay 

 pits, bluffs, or special excavations (Fig. 2) are smoothed with 

 spade and brick trowel, and the series of varves are measured 

 from the bottom, if possible, by marking the varve limits care- 

 fully on narrow strips of strong paper (Fig. 3). The field measure- 

 ments are transformed into curves or graphs on paper ruled with 

 lines spaced exactly five millimeters apart, since experience has 

 proved that to be the best. The thickness of the lowest varve 

 is set off on the first vertical line, the second varve on the next 

 line to the right of the former, and so on, and the points marking 

 the thickness of the varves are connected (Fig. 4). 4 The graph 

 thus secured is compared with one from another locality by 

 moving the two until they match, which, as already explained, 

 they do if they have a series of varves in common. If the bottom 

 varve at one locality is found to correspond to varve number 

 37 at a more southerly locality, the ice border evidently 

 withdrew over this place 36 years earlier than over the former 

 place. 



If the varves are very thin or for some other reason difficult 



4 De Geer. in order to emphasize the stratigraphy, originally plotted the varves 

 on a series of horizontal lines, beginning with the lowest line and measuring in 

 from the left edge of the paper. When it proved inconvenient to compare the 

 graphs in that position, he turned them oo° but kept the sequence of the varves; 

 so that on his graphs the first or lowest varve is to the right. 



