THE DATE OF THE GLACIAL PERIOD. 547 



entering Chicago from the east, but they are limited in extent, 

 and are in process of formation at the present time, the rate 

 of which can be approximately calculated. Lake Michigan 

 is now a closed body of water at the south end, and it is eating 

 into its western banks and bluffs at a rapid rate, the shelf 

 eroded by the waves since the departure of ice from its central 

 depths being about seven miles wide. The shingle, gravel 

 and sand derived from this erosion of the banks is carried along 

 the west shore southward past Chicago to the south end of 



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Fig. 140 — Hypothetic hydrography of the Great Lakes at a date after the melting of the 

 great glacier from the St. Lawrence Valley. 



the lake, where it is taken up by the wind and blown outward 

 to form the dunes which are now so prominent a feature in 

 the landscape. It has been important for various reasons to 

 learn how fast the sand is being carried past the Chicago 

 front, so that engineers have made very careful estimates. On 

 comparing the rate at which the material is being carried 

 past Chicago, with the total amount of sand contained in the 

 dunes at the south end of the lake, it appears that the process 

 cannot have been going on more than 10,000 years. 



Now, since Dr. Upham estimates that the dunes at the 

 south end of glacial Lake Agassiz are not over one-tenth the 



