THE DATE OF THE GLACIAL PERIOD, 573 



into a pyramid seventy-seven feet high.) (Z>) Then, descending 

 fifty and five tenths feet vertically, you are carried one hundred 

 and thirty-eight feet horizontally, reaching at that point the 

 edge of a circular mass of peat which is ninety-six feet in di- 

 ameter, (c) From the opposite side the ascent of the northern 

 rim begins, and you descend from its top to the valley, repeat- 

 ing almost exactly the first descent from the pond. The dis- 

 tance from rim to rim, or the diameter is three hundred and 

 eighty feet. 



It is evident that since the first formation of this crater- 

 shaped depression no material can have reached the bottom, 

 except from three sources : 1. The wash from the side ; 2. 

 The decay of vegetation which grew within the circumference 

 of the rim ; 3. The material brought by the winds. It is 

 equally evident that what is once in can not get out. 



Dust, leaves, and twigs carried by the winds inevitably 

 lodge in such depressions more thickly than in other places, 

 since the atmosphere in such hollows is comparatively quiet. 

 For the same reason the surrounding trees as they are blown 

 down are more likely to fall toward the center of the kettle- 

 hole ; and the ashy material which their roots abstract from 

 the sides of the depression is no insignificant factor in the 

 problem. 



Now. from the angle of the declivity, the original depth of 

 the depression can be approximately estimated. If the angle 

 be still the same as at first, the first three terms of the propor- 

 tion would be 138 : 50*5 : : 48 : 17-Jf, making the original 

 depth below the present surface of the peat a trifle over 17*5 

 feet. If, however, we suppose the original slant to have been 

 steeper and the rim higher, we can still see that there must 

 have been a limit to the depth. Suppose the rim to have been 

 one third higher and the slant one third steeper, we then 

 should have in round numbers the proportion 138 : 68 : : 48 : 

 23ff. making the original depth of the depression nearly 

 twenty-four feet below the present surface of the peat. From 

 the nature of the material it is impossible that the depth could 

 originally have much exceeded that amount. 



Accepting this conclusion, the problem is, to determine the 

 time it would require the agencies mentioned above to fill the 



