RATE OF RECESSION 79 



icebergs which were broken off in the glacial lake can hardly have 

 been able to push together a clay bed so thick. That a landslide 

 was the cause is equally improbable, since the ground towards 

 the west rises only 60 feet in the first half-mile, where it reaches 

 the foot of a drumlin whose crest is only 140 feet above the local- 

 ity. The last possible explanation, that the ice edge readvanced, 

 pushed together the clay, and deposited the till cover, also meets 

 some difficulty. The readvance would have occurred about year 

 4800, i.e. at a time when deposition of clay proceeded undis- 

 turbed at Northampton, 7 miles to the west. Since locality 28, 

 the ice edge at which lay 16.8 miles (27.1 km.) north of the ice 

 edge when at locality 24, was uncovered by the ice before year 

 5084, the annual recession of the ice from 24 to 28 must have 

 averaged about 328 feet (100 m.). This fact, however, forms 

 no obstacle. Within less than 220 years after the halt at Clare- 

 mont, N. H., the annual recession amounted to 615 feet (188 

 m.; see p. 76), and in Finland Sauramo (1918, p. 31) found that 

 the rate of retreat was most rapid just after a halt or a slow 

 recession. 



At locality 20, situated at the northwestern edge of Northamp- 

 ton, the following section was measured, beginning at the top: 



3 feet sand. 



2 feet weathered and disturbed varve clay. 



\% feet clay, no varves, not connected with the normal curve. 



13 feet silty clay, homogeneous through disturbance. 



4 feet quicksand, slidden at the top, varves 4633 and 4634. 

 Silty clay, sandy at the top, varves 4377-4632. 



Depth to bottom unknown. 



Figure 16 shows in one-third actual thickness varves 4560-4634 

 at this locality and, for the sake of comparison, varves 4560-4642 

 at locality 19, situated \}4 miles to the south. The most interest- 

 ing thing about profile 20 is the increase in thickness of the 

 varves beginning with number 4586. Varve 4587 and those 

 from 4592 upward consist of quicksand. Below number 4586 the 

 layers have about the same thickness as at the other localities 

 in the region but consist of somewhat coarser material. The 



