THE DATE OF THE GLACIAL PERIOD. 571 



Again evidence comes from the extent to which lakes, 

 dating from the Glacial period, have been filled with sedi- 

 ment. Little reflection is required to make it evident that 

 our present lake-basins could not always have existed ; for, 

 except where counteracting agencies are at work, the " wash " 

 of the hills will, in due time, fill to the brim all inclosed 

 areas of depression. Mr. Upham, of the Minnesota Geo- 

 logical Survey, expresses surprise at the small extent to 

 which the numerous lakes of that State have been filled 

 with the sediment continually washing into them. " The 

 lapse of time since the Ice age has been insufficient for rains 

 and streams to fill these basins with sediment, or to cut out- 

 lets low enough to drain them, though in many instances we 

 can see such changes slowly going forward." * 



Dr. E. Andrews, of Chicago, has made calculations, de- 

 serving of more attention than they have had, concerning the 

 rate at which the waters of Lake Michigan are eating into 

 the shores, and washing the sediment into deeper water or 

 toward the southern end of the lake.f The United States 

 Coast Survey have carefully sounded the lake in all its parts, 

 and have ascertained the width of the area of shallow water 

 extending inward from the shores. It is well known that 

 waves are limited in their downward action, so that there 

 will be a surrounding shelf, or shoulder of shallow water, in 

 cases where the waves of a deep lake are eroding its banks. 

 This fringe of shallow water encircling Lake Michigan is 

 only a few miles wide ; and from such data as have been gath- 

 ered, the average rate of erosion is found to be as much as 

 five or six feet per annum ; which would indicate that the 

 lake-basins had not been in existence more than seventy-five 

 hundred years. 



Leaving these more indefinite and in many respects un- 

 satisfactory efforts to estimate the age of lake-basins, we 

 may get some assistance in approximating to a correct chro- 



* " Minnesota Geological Report" for 1879, p. 73. 



f "American Journal of Science," vol. xcviii, 1869, pp. 172 et seq. 



