WATER-PLANES AND CRUST AL DEFORMATION 351 



in different stages."^ Much more important, however, are the 

 changes in level that occur over a period of years rather than months. 

 The record of the fluctuations of lakes Michigan and Huron from the 

 year 1800 to 1899^ is most instructive upon this point. The maxi- 

 mum change of level shown is 6 feet, while the average fluctuation 

 between successive high and low stages, for all changes over i foot, is 

 about 2.5 feet. What then is the true water-level? The question 

 has a very direct bearing on the problem of fixing the position of the 

 water-planes of the glacial Great Lakes from observations upon their 

 shore-lines, since one beach formed in one decade might well be 

 several feet higher or lower than a second constructed at another 

 point in a following decade. Lane has noted the effect of such fluctua- 

 tions in speaking of a 14-foot beach ridge in Huron County, Michigan. 

 He says: "The water-line probably lay a little lower, at 10 or 11 feet 

 above the 582-foot datum (the present lake level), at a time perhaps 



within range of tradition The high water-line of 1886 is well 



marked in a crest from 4 to 7 feet above the present lake level. "^ 

 The idea naturally suggests itself that a reconstructed water-plane, 

 when drawn on a large enough scale, instead of being represented as a 

 single line, should be shown as a zone of such a width as would take 

 account of these changes of level. 



As an extreme case, it may be supposed that at a time of maximum 

 high water a storm beach was built up 6 feet above the water-level, 

 while at a time of low water a terrace was cut 2 feet below the lake 

 level. There would be a difference of 14 feet (8 in beach and terrace 

 +6 in water-level) between the two, yet they would belong to the 

 same period and might have been formed within 25 years of each 

 other. Such an explanation may account for the discrepancy (of 

 10 or 12 feet) between a cut terrace and bowlder beach on the north , 

 shore of Lake Superior which Taylor believed belonged to the same 

 shore-line. He says: "Measurements of the same shore-line often 

 vary 7 or 8 feet, sometimes more, from this cause"'* (one measure- 



^ W. Uphani) "The Glacial Lake Agassiz," U. S. Geological Survey, Monograph 



25y P- 277, 1896. 



» Lane, op. cit., Plate V. 3 Op. cit., p. 76. 



4 F. B. Taylor, "The Nipissing Beach on the North Superior Shore," American 

 Geologist, Vol. XV (1895), p. 307. 



