70 ICE RECESSION IN NEW ENGLAND 



The extra material in the varves was picked up by the vigorous 

 drainage river along its course. Drainages observed in the clay 

 deposits in New England amount to more than half a hundred. 

 They are rare in the southern part, but numerous from northern 

 Massachusetts northward, where the relief becomes stronger. 

 All drainage varves found are treated under the description of 

 the normal curve on pages 49-63. Although it is desirable to 

 get rid of them in the normal curve, the attempt to do this has 

 not always succeeded. To distinguish them they have been 

 marked by dashed lines. What follows here is only a general 

 account. 



While the thickest drainage varve observed, varve 6808, at 

 locality 67, Hanover, N. H., is 12 feet (3.5 m.), many of them 

 are so thin that they are not suspected to be abnormal before 

 they are found to correspond to thin varves in other valleys. 

 This source of mistakes must be kept in mind when using the 

 varve curve as a thermograph and makes measurements in dif- 

 ferent lakes the more desirable. 



Most drainages were inconsiderable and took place during a 

 single year. A great many of them, however, occurred during a 

 number of summers, though generally less than five. Drainages 

 of two and three years are common. In those lasting two years 

 the amount of sediment brought during the first year was in 

 most cases the greater. In drainages continuing for three or 

 more summers the amount of sedimentation usually increased 

 during the first one or two years, reaching a maximum at or 

 before the middle of the period of drainage. Occasionally the 

 first or the last drainage varve is the thickest. 



Since the drainage layers grew in thickness towards the mouth 

 of the river by which the ponded lake discharged, the position of 

 this lake can be traced by their help. While so far no detailed 

 studies have been carried out to determine the exact position 

 of ice lakes, the valleys in which they were ponded or through 

 which the discharge took place are in several cases known. Thus 

 Figure 1 4 shows three drainages which apparently came into the 

 Connecticut above its junction with the Passumpsic, since the 



