456 THE ILLINOIS GLACIAL LOBE. 



breadth of the lake, and return currents must be at some depth. On 

 the other hand, a prevailing south wind, such as occurs for short periods 

 during the summer, will induce a northward movement across the entire 

 breadth of the lake. The contours of the shore seem to favor a northward 

 movement from direct west winds in the north half and a southward move- 

 ment in the south half of the lake. As the prevailing winds are often from 

 the west, these become the most protracted of the movements of surface 

 water. Cooley has found that breakwaters along the shore support this 

 interpretation. In the southern half of the lake they are largely con- 

 structed to protect the harbors from the drift on the north side, while in 

 the northern half they are constructed to protect them from drift coming 

 from the south. In view of this apparently changeable course of lake 

 movements, it seems doubtful if estimates, such as Dr. Andrews attempted, 

 have the value that some have attached to them. 



Dr. Andrews also made an estimate of the age of the lake from the 

 annual amount of destruction of the bluffs. To determine the rate of 

 erosion on the west coast of Lake Michigan, he gathered a large number of 

 observations, mostly derived from surveys, and after rejecting loose or 

 vague estimates, as well as erosions brought to notice because of remark- 

 able rapidity, he obtained the results given in the following table: 



Erosion of the shore of Lake Michigan per annum. 



Feet. 



Evauston 16. 95 



Old pier, 2 miles farther north 4.90 



One mile farther north 3. 08 



Winn etka 4-05 



One mile farther north 6. 05 



Lake Forest 1-65 



Waukegan 0.00 



Two miles farther north 0. 00 



State line 16.50 



Kenosha 12.00 



Two miles farther north 3. 00 



Three miles farther north 12.00 



Racine Point 16 00 



Eacine 6.00 



Oak Creek 2.00 



One mile farther north 1. 60 



Milwaukee 6. 25 



Port Washington 2. 30 



One mile farther north 1. 50 



Place farther north ". 3.00 



Place 4 miles south of Sheboygan 8. 00 



Sheboygan 6. 25 



Manitowoc 5. 00 



