84 ICE RECESSION IN NEW ENGLAND 



15 miles (24 km.) east of here, which have been mapped by 

 Goldthwait (19 16). As for the length of time represented by the 

 halt, 280 years may be a minimum figure. 



Rate of Retreat and Type of Clay 



Difference in thickness between varves is usually due to varia- 

 tion of the coarse summer layers, the winter layers being almost 

 equally thick from year to year. Accordingly, thin varves con- 

 tain a greater percentage of fine material than do thick ones. 

 The thickness of the varves, the amount of sedimentation, is a 

 measure of the amount of melting. Since slow recession and 

 halt of the ice border, as observed by De Geer long ago, are 

 usually recorded by thinner and more greasy varves than is 

 fast recession, it is evident that the retardations were character- 

 ized by little melting, in other words, by low temperature (cf. 

 p. 86). Thus the clay gives an idea of the rate of the retreat and 

 of the climatic conditions, especially in regions which, like 

 Fenno-Scandia, are built up of hard, coarse Archean rocks (see 

 also Sauramo, 1918, p. 37). This rule, however, can be applied 

 only with the greatest caution to districts occupied by both hard 

 and soft rocks. A soft, fine-grained slate or schist evidently 

 gave rise to a fatter clay than did a hard, coarse-grained granite, 

 regardless of the amount of melting. There occur, therefore, 

 in New England clays deposited during rapid recession consisting 

 exclusively of such fine material that the varve limits cannot be 

 distinguished. 



Another reason why the thickness and texture of the varves 

 cannot be used as indicators of the ice retreat in New England is 

 the fact that, in the small lakes which occupied the valleys, 

 thick and thin varves often only mean large or small drainage 

 area. It is particularly significant that during an annual reces- 

 sion of more than 1,000 feet (300 m.) comparatively thin and 

 fat clay varves were deposited in the Connecticut Valley at 

 Woodsville, while at locality 63, 50 miles farther from the ice 

 front but below important tributaries, the same varves are 

 silty and three to four times as thick. 



