GEOLOGICAL AND NATURAL HISTORY SURVEY. 85 



occupied its present level. Where the shores are of drift clay the 

 terrace generally has a breadth of from 2 to 6 miles, but at the 

 south end of Lake Michigan it is nearly ten miles. The west shore 

 of Lake Michigan was examined in some detail between Chicago 

 and Manitowoc and found to have an average width of 3.98 miles. 

 This terrace slopes gently outward to the depth of about 60 feet, 

 when the bottom dips more suddenly to the deep water of the 

 basin. It is thought by Dr. Andrews to be the product of wave 

 action and is denominated by him the terrace of erosion. The pres- 

 ent writer, however, has no opinion concerning its origin. The time 

 required for the formation of this terrace was computed by using 

 the average width of the terrace as a dividend and the annual rate 

 of erosion as a divisor. As the outer edge of the terrace is at the 

 depth of 60 feet, the position of the old shore was assumed to be at 

 a point where a line drawn from the top of the present blufif of the 

 lake to the outer edge of the terrace would meet the surface of the 

 lake. These estimates give the average position of the old shore, 

 a distance of 2.72 miles from the present shore. Dividing this 

 distance by the annual rate makes the total age of the terrace 2,720 

 years, or a duration nearly the same as that computed by the drift^ 

 ing of the sand. If the rate of erosion determined by the Wiscon- 

 sin Survey be substituted the age would be 4,708 years. 



The estimate based upon the rate of erosion of the shore of 

 the lake is probably much more reliable than that based upon the 

 drifting of the sand past the piers, but the great variability in the 

 height of the shore (from 10 feet up to 100 feet or more) and the 

 variability in the rate of recession (from o to 16.95 feet per year) 

 make it evident that the above computation is at best only a rude 

 approximation. These. estimates serve, however, as a provisional 

 measurement of the duration of this stage of the lake and have 

 much value in its bearing upon the length of postglacial time. Dr. 

 Andrews remarks that they are useful, in showing that it is impos- 

 sible to allow, even on the most liberal estimates, any such post- 

 glacial antiquity as 100,000 years, which has often been claimed. 



ACKNOWLEDGEMENTS. 



In the preparation of this paper and its illustrations, I am 

 happy to acknowledge the aid kindly given by many persons, 

 among whom should be mentioned Prof. Oliver Marcy, Prof. L. 

 E. Cooley, Prof. W. K. Higley, and, most of all, Prof. T. C. Cham- 

 berlin, the President of this Academy. In the preparation of 



