THE DATE OP THE GLACIAL PERIOD. 347 



Michigan City. This equals 129,000 cubic yards for a year, 

 which can scarcely be more than one quarter or one fifth of 

 the total amount in motion. At this rate, the sand accum- 

 ulations at the southern end of the lake would have been 

 produced in a little less than seven thousand years. 



" If," says Dr. Andrews, " we estimate the total annual 

 sand-drift at only twice the amount actually stopped by 

 the very imperfect piers built — which, in the opinion of 

 the engineers, is setting it far too low — and compare it 

 with the capacity of the clay-basin of Lake Michigan, we 

 shall find that, had this process continued one hundred 

 thousand years the whole south end of Lake Michigan, 

 up to the line connecting Chicago and Michigan City, 

 would have been full and converted into dry land twenty- 

 five thousand years ago, and the coast-line would now be 

 found many miles north of Chicago." * 



It is proper to add a word in answer co an objection 

 which may arise in the reader's mind, for it will doubtless 

 occur to some to ask why this sand which is washed out 

 by the waves from the bluffs is not carried inward towards 

 the deeper portion of the trough of the lake, thus pro- 

 ducing a waste which would partly counteract the forces of 

 accumulation at the south end. The answer is found in 

 the fact that the south end of Lake Michigan is closed, 

 and that the currents set in motion by the wind are such 

 that there is no off-shore motion sufficient to move sand, 

 and, as a matter of fact, dredgings show that the sand is 

 limited to the vicinity of the shore. 



By comparing the eroded cliffs upon Michigan and the 

 other Great Lakes with what occurs in similar situations 

 about the glacial Lake Agassiz, we obtain an interesting 

 means of estimating the comparative length of time occu- 

 pied by the ice-front in receding from the Canadian bor- 

 der to Hudson Bay. 



* Southall's Recent Origin of Man, p. 502. 



