THE PBESENT BEACH OP LAKE MICHIGAN. 455 



Dr. Andrews attempted to estimate the length of time involved in the 

 accumulation of the beach deposits by measuring the amount of sand carried 

 southward past the piers at Chicago and Michigan City. The sand annually 

 stopped by the two piers was found to be 129,000 cubic yards. If this 

 represented the whole drift past the piers, the period required for the accu- 

 mulation of the sand in all the beaches would be about 26,000 years and 

 the duration of Lake Michigan at its present stage would be 13,000 years. 

 He estimates, however, that not more than one-fourth or one-fifth of the 

 southward-drifting sand is stopped by the piers, and thus reduces the period 

 to less than 6,000 years, with but about 3,000 years for Lake Michigan. 



Dr. Andrews's estimates were based on the assumption that there is a 

 southward-flowing current on each side of the lake, carrying sand to its 

 present head. Investigations made by the Weather Bureau in 1892 and 

 1893, under the direction of Prof. Mark Harrington, 1 led him to the conclu- 

 sion that the currents on the east shore in the southern portion of the basin 

 are northward instead of southward. He accounts for the accumulation of 

 sand on the north side of breakwaters along this coast by the action of the 

 surf, in storms blowing from the north, which is more transient than the cur- 

 rents proper and would affect the southern part of Lake Michigan only 

 when the wind was in the north. • This occasional phenomenon is very effi- 

 cient when it occurs. He concludes that the estimates of time involved in 

 the formation of beaches have less value than they would have if the accu- 

 mulation were due more largely to lake currents. 



Considerable study of the movement of water in Lake Michigan has 

 been made by the Chicago Drainage Commission, largely under the direc- 

 tion of Professor Cooley. As a result of these investigations, which involve 

 not only a study of bottle papers, but also a thorough canvass of the 

 opinions of lake captains and an examination of breakwaters, Cooley has 

 reached the conclusion that the effective work on the shores is due to waves 

 and not to currents, and it is a matter of doubt if this lake has such a sys- 

 tem of currents as are indicated by Professor Harrington's charts. The 

 movement of the water seems to depend mainly upon the wind, but is 

 governed to some degree by the contours of the shores. If the north 

 winds prevail for a few days, as is often the case in the spring months, 

 the surface water appears to have a southward movement throughout the 



1 Currents of the Great Lakes as deduced from the movement of bottle papers during the seasons 

 of 1892 and 1893, by Mark W. Harrington : Weather Bureau Bulletin B, U. S. Dept. of Agriculture, 1894. 



