THE PRESENT BEACH OF LAKE MICHIGAN. 459 



The measurements by Mr. Glavin show but little more erosion than 

 those reported by the Wisconsin survey. In view of these measurements 

 the rate for the entire shore is probably scarcely more than half that of Dr. 

 Andrews's estimates. 



Dr. Andrews calls attention to the existence of a submerged terrace, 

 which, he thinks, furnishes a ready means for determining approximately 

 the original position of the shore, and consequently the distance which the 

 bluffs have receded since the water 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 10 miles. The west 

 shore of Lake Michigan was examined in some detail between Chicago and 

 Manitowoc and the terrace was 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 rapidly 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 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 bluff 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 drifting of the sand. If the rate of erosion determined 

 by the Wisconsin 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 to 16.95 feet per year) makes 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 impossible to allow, even 

 on the most liberal estimates, any such postglacial antiquity as 100,000 

 years, which has often been claimed. 



