364 ARTHUR C. TROWBRIDGE 



an average thickness of 100 feet (probably less than the actual 

 thickness) for the first mile back from its edge. To estimate 

 the total annual water supply from the ice, it is necessary to make 

 reasonable supposition as to the amount of annual melting measured 

 in a horizontal line normal to the front of the ice. The ice front 

 may be considered to have been essentially stationary, and the 

 amount of ice melted can be measured by the rate of glacial motion 

 in the marginal portions of the ice. Chamberlin and Salisbury 1 

 estimate that the ice in the Greenland glacier moves something less 

 than a foot a week near its edge. Suppose that the glacier which 

 affected Devils Lake moved 6 inches a week or 26 feet a year. If, 

 then, it be assumed that a volume of ice 11. 6 miles long measured 

 along the edge of the glacier, 100 feet thick measured vertically, 

 and 26 feet wide measured normally to the ice front, melted each 

 year and ran into Devils Lake, there would be an annual total of 

 about 1^ billion cubic feet. The decreased volume due to change 

 from ice to water may be neglected in a computation where there 

 are so many assumptions and where all figures have been reduced 

 to a minimum for safety. 



To estimate the capacity of the basin of Devils Lake during the 

 occupancy of the ice, it is necessary to have its length, width, and 

 depth. Measured from moraine to moraine around the curve 

 of the gap, the basin is almost exactly 2 miles long. The average 

 width from end to end and from lake level to lowest point in the 

 rim of the basin is approximately ^ mile. The depth of the glacial 

 lake may be found by subtracting the altitude of the present lake 

 bottom from the altitude of the lowest point in the rim of the basin, 

 neglecting the glacial debris below the bottom of the lake, which 

 would have displaced the water as it was deposited. Computed 

 in this way, the maximum depth of the glacial lake was 270 feet. 

 Multiplying the depth, width, and length, the capacity of the 

 glacial basin was about 7^ billion cubic feet. 



According to these figures the water from the glacier would have 

 filled the basin of Devils Lake to overflowing in about five years. 

 If the rate of advance of the ice was greater than assumed above 



1 T. C. Chamberlin and R. D. Salisbury, Geology, I, 261. 



