THE NORMAL CURVE 49 



have been matched represent a retreat after the long halt at 

 the Fenno-Scandian moraines. Furthermore, the ice retreat 

 in New England does not show agreement with that in southern 

 and central Sweden. Accordingly, and because I hope to be able 

 to extend the curve southward, the lowest varve measured at 

 Hartford has simply been given the arbitrary number 3001. 

 The numbers of the varves every ten years from 3001 to 7400 

 are marked on the base lines of the graphs. For the sake of 

 convenience the curve has been broken every 200 years. 



So far as there is material for it, then, the normal curve gives 

 the average thickness of the annual clay layers during the 

 retreat of the last ice sheet across the greater part of New England. 



Reliability and Significance of the Varves Measured 



In the following the reliability of the varves measured, from 

 3001 to 7400, is discussed by groups of varves, and comment is 

 made on the agreement of curves covering the same period 

 measured at different localities. Within a group individual 

 varves of special significance are discussed. 



3001-3449. — Very good. The ice front was already fairly distant 

 when the lowest of these varves were deposited in 

 the Hartford region. Accordingly, the sedimenta- 

 tion took place under very favorable conditions, 

 and the varve curves from the different localities 

 show a striking correspondence. The thickest 

 varves are silty, and some of them, as for instance 

 3444, perhaps represent drainages of ice-ponded 

 lakes. 

 3188 and 3224, at locality 5, contain quicksand and 

 measure \]/i and i^i inches (3 and 4 cm.) respec- 

 tively. They perhaps mark drainages. 



3450-3500. — The Hartford curve ("3 Conn.," PI. I) is good, while 

 the Springfield curve ("3 Mass.") is less reliable, 

 since the varves are often sandy. The correspond- 

 ence of the two graphs is fairly good. 



