346 MAN AND THE GLACIAL PERIOD. 



shores and washing the sediment into deeper water or 

 towards the southern end of the lake. With reference to 

 the erosion of the shores, it appears from the work of the 

 United States Coast Survey that a shoulder, covered with 

 sixty feet of water, representing the depth at which wave- 

 action is efficient in erosion, extends outward from the 

 west shore a distance of about three miles, where the 

 sounding line reveals the shore of the deeper original 

 lake as it appeared upon the first withdrawal of the ice. 



From a variety of observations the average rate at 

 which the erosion of the bluffs is proceeding is found to 

 be such that the post-glacial time cannot be more than 

 ten thousand years, and probably not more than seven 

 thousand. 



An independent mode of calculating this period is 

 afforded by the accumulations of sand at the south end of 

 the lake, to which it is constantly drifting by the currents 

 of water propelled against the shores by the wind ; for the 

 body of water in the lake is moving southward along the 

 shores towards the closed end in that direction, there being 

 a returning current along the middle of the lake. All 

 the railroads approaching Chicago from the east pass 

 through these sand deposits, and few of the observant 

 travellers passing over the routes can have failed to no- 

 tice the dunes into which the sand has been drifted by 

 the wind. Now, all the material of these dunes and sand - 

 beaches has been washed out of the bluffs to the north- 

 ward by the process already mentioned, and has been 

 slowly transferred by wave-action to its present position. 

 It is estimated that south of Chicago and Grand Haven, 

 this wave-transported sand amounts to 3,407,451,000 cubic 

 yards. This occupies a belt curving around the south end 

 about ten miles wide and one hundred miles long 



The rate at which the sand is moving southward 

 along the shore is found by observing the amount annu- 

 ally arrested by the piers at Chicago, Grand Haven, and 



