THE DATE OF THE GLACIAL PERIOD. 345 



amount of deposition and vegetable accumulation which 

 had taken place in a kettle-hole near Pomp's Pond, in 

 Andover, Mass. The diameter of the depression at the rim 



Fig. 107. — Section of kettle-hole near Pomp's Pond, Andover, Massachusetts (see 

 text). (For general view of the situation, see Fig. 30, p. 78.; 



was 276 feet. The inclination of the sides was such that 

 the extreme depression of the apex of the inverted cone 

 could not have been more than seventy feet; yet the 

 accumulation of peat and sediment only amounted to a 

 depth of seventeen feet. The total amount of material 

 which had accumulated would be represented by a cone 

 ninety-six feet in diameter at the base and seventeen feet 

 at the apex, which would equal only a deposit of about 

 five feet over the present surface of the bottom. It is 

 easy to see that ten thousand years is a liberal allowance 

 of time for the accumulation of five feet of sediment in 

 the bottom of an enclosure like a kettle-hole, for upon 

 examination it is clear that whatever insoluble material 

 gets into a kettle-hole must remain there, since there is 

 no possible way by which it can get out. Now five feet is 

 sixty inches, and if this amount has been six thousand 

 years in accumulating, that would represent a rate of an 

 inch in one hundred years, while, if it has been twelve 

 thousand years in accumulation, the rate will be only one 

 two-hundredth of an inch per year, a film so small as to be 

 almost inappreciable. If we may judge from appearance, 

 the result would not be much different in the case of the 

 tens of thousands of kettle-holes and lakelets which dot 

 the surface of the glaciated region. 



In the year 1869 Dr. E. Andrews, of Chicago, made 

 an important series of calculations concerning the rate 

 at which the waters of Lake Michigan are eating into the 



