CHAPTER IV 

 THE NORMAL CURVE 



Method of Construction 



The curves I to 21 (Pis. I-V), form together a normal curve 

 of 4,400 years which, with the exception of a gap of 200 to 300 

 years at Claremont, N. H., records the recession of the last ice 

 sheet from Hartford, Conn., to the region of St. Johnsbury, Vt. 

 To construct it, all individual curves were first matched and 

 corrected for numbers of varves. If, for example, out of four 

 measurements three agreed but one had one varve less or more 

 than the others, the exact location of the mistake was determined 

 and the curve was corrected by dividing one varve in two or 

 uniting two varves in one, so that this curve agreed with the 

 three others. In order to get the number of the varves accurately, 

 as many field measurements were made as time and conditions 

 permitted. Then the curves or such parts of them as included 

 undisturbed varves of normal variation and thickness were 

 selected for constructing the normal curve. To this end the 

 graphs were compared as to thicknesses of the varves and as to 

 fluctuations; and those curves were discarded which showed 

 great difference in thickness from the majority or poor agreement 

 in the shape of the curve. Having for the greater part of the 

 normal curve much material at my disposal, I generally went 

 rather far in discrimination, so that except for too thick or too 

 thin varves many discarded series show excellent correspondence 

 with the normal ones. The normal curve was constructed from 

 the selected individual curves by calculating the average thick- 

 ness of each single varve. Some varves, however, are abnormal 

 at all localities examined and so have been marked in the normal 

 curve by broken lines (cf. p. 70). 



