566 THE ICE AGE IN NORTH AMERICA. 



compelling the engineers to open a new channel across an 

 undisturbed neck of the original glacial till. The section of 

 this chosen for observation was 500 feet long, and at first 

 consisted of a ditch twenty-one feet wide at the top, and 

 ten at the bottom, with an average depth of eleven feet, 

 though on the south side it rose to a height of twenty feet 

 above the bottom. But, after a lapse of twelve years (in 

 1907), the stream had enlarged the ditch to a width of fifty- 

 one feet at the top, and of seventeen feet at the bottom, giving 

 an average width of thirty-four feet compared with the original 

 of fifteen and one-half feet. A simple mathematical calcula- 

 tion shows, therefore, that in twelve years this stream had 

 removed from a 500-foot section whose banks were exposed 

 to the direct action of the current on both sides, 101,750 

 cubic feet of solid matter, or, 8,450 cubic feet per annum. 



To get a more perfect basis of comparison, measurements 

 were taken of a section 5,000 feet long below the village, where 

 the original conditions had been undisturbed. In this sec- 

 tion the eroded trough averaged 400 feet in width and seven- 

 teen feet in depth, and this entirely in glacial till such as 

 characterizes the whole valley. The total amount, therefore, 

 of work accomplished, by the stream in this section since the 

 present line of drainage was opened was the removal of 

 34,000,000 cubic feet of till. 



To obtain a still more approximate basis for calculation 

 it was necessary to measure the length of the sections of the 

 edge of the trough where the stream impinges directly against 

 the bluff and so is eroding under conditions similar to those 

 in the cut-off at the reservoir. Upon doing this it was 

 found that these exposed sections amounted to 1,600 feet in 

 length, which is 600 feet more than that of both sides of the 

 cut-off. The annual erosion, therefore, in the 5,000-foot 

 section is now one and six-tenths greater than in the cut-off, 

 making 13,568 cubic feet of material per year. At this rate 

 the 34,000,000 feet of material from the 5,000-foot section 



