324 ILLINOIS ACADEMY OF SCIENCE 



cliffs, Dr. Andrews, in 1870, found that tlie average re- 

 treat of the lake cliffs between Evanston, 111., and Mani- 

 towoc, Wis., a distance of 180 miles, was 5.28 feet per 

 year. The measurements on which his estimate was 

 based ranged through a period of from 10 to 35 years.* 

 In 1870, or thereabouts, he found the retreat of the cliffs 

 at Kenosha to be as much as twelve feet in a year. In 

 1874, measurements furnished Dr. Chamberlin showed 

 that at one place in Eacine, Wis., the cliffs for 24 years 

 had been receding at a rate of 9.73 feet per year. 



Measurements recently completed near Kenoska in- 

 dicate that the cliffs have retreated as much as 34 feet 

 locally within a period of one year and seven months. 

 The average retreat during this period, as indicated by 

 eight measurements was 27.7 feet. The maximum retreat 

 was found to be 34.2 feet; the minimum, 22.6 feet. The 

 eight measurements mentioned above were taken within 

 a distance of 2.5 miles. Taking the average retreat as a 

 fair indication of the loss of land, the figures given repre- 

 sent the loss of a little more than eight acres within two 

 years. The height of the cliffs ranges from 20 to 40 

 feet. The material of the cliff's is partly till, and partly 

 stratified sands and clays deposited by Lake Chicago. 

 The annual retreat of the cliffs as indicated at present 

 approximates 17 feet as compared with the 12 foot re- 

 treat noted by Andrews. 



That this rapid retreat of the cliffs is responsible for 

 a marked diversion of the stream is shown in the fol- 

 lowing fashion. As the stream gradually approaches 

 the lake, the continuity of its eastern valley wall is 

 broken in two places. The breaks or breaches occur at 

 places where the valley swings in meandering curves to- 

 wards the lake. Through these wide breaches it is pos- 

 sible to obtain an open view of the lake from within 

 the valley of the river. At the breaches the beach of 

 Lake Michigan is built directly upon the flood plain of 

 the river. Opposite the northernmost breach the river 

 flows but fifty feet distant from the lake. Merely the 



^Andrews, Dr. Edmund, The North American Lakes Considered As 

 Chronometers of Post Glacial Time, Chicag-o Acad, of Sci., Vol. II, 1870, p. 7. 



