CHAPTER VII 



THE RATE OF RECESSION AND CONDITIONS 

 CONTROLLING RECESSION 



On account of the very considerable depth of the varve 

 sediments in the New England valleys their bottoms have been 

 reached only at a limited number of localities, almost all of 

 which are situated in New Hampshire and Vermont. Here the 

 rate of retreat, accordingly, has been exactly determined in 

 several instances, while that has not been possible in Connecticut 

 and Massachusetts. However, the bottom of the clay has in 

 many cases been almost reached, enabling an approximate 

 determination of the speed of the retreat. 



On the map, Plate VI, the positions of the ice edge have been 

 marked for every- ioo years. Which of these positions are deter- 

 mined by bottom varves may be deduced from their location with 

 reference to the localities indicated in Chapter III at which 

 bottom varves were reached. Accelerated retreat between two 

 points indicated by the speed south and north of them has been 

 shown on the map by putting the ioo-year lines at different 

 distances, so as to illustrate as truly as possible the gradual 

 increase. Retardation in the recession has been marked in a 

 corresponding way. 



The extent of the examined field, which reaches from Hartford, 

 Conn., to St. Johnsbury, Vt., is 185 miles (298 kilometers). 

 The retreat across this belt is registered by a continuous series 

 of annual clay layers except for a narrow gap at Claremont, N. H., 

 probably representing 200 to 300 years. The time occupied by 

 the recession from Hartford to St. Johnsbury was about 4,100 

 years. This makes an average rate of 22 years to a mile, or of 

 238 feet (73 meters) a year. 



The following table gives the rate of retreat of the ice border 



