454 THE ILLINOIS GLACIAL LOBE. 



encroachment. Along the borders of Hyde Park, in the south part of Chi- 

 cago, the lake is building a beach and is tending to fill in rather than to 

 extend the lake in that region. In Indiana the lake is filling in rather than 

 extending its borders. In southwestern Michigan it is eroding the promi- 

 nent parts more rapidly than the bights, thus giving the lake a more regular 

 outline. In this connection it may be remarked that the tendency generally 

 is to remove the prominent parts and fill the bights. 



Dr. Andrews computed the bulk of the beach as follows: For 25 miles 

 west from Michigan City it maintains an average cross section of about 

 6,000 square yards, and its contents are 264,000,00U cubic yards. In this 

 division the beach is in the form of a lofty belt of sand dunes, about one- 

 third of a mile wide and in places nearly 200 feet in height. In the next 

 8 miles west the beach spreads out into a broad belt of low parallel ridges 

 about two miles in extreme width. This division has a cross section of 

 about 16,000 square yards, after deducting the sand which was deposited 

 by Lake Chicago. Its contents amount to 225,280,000 cubic yards. From 

 the Indiana line, near Wolf Lake, to Chicago River, a distance of 16 miles, 

 the sand occupies a belt estimated to be 7 yards thick on the shore but 

 running out to a thin edge at the average distance of 2,500 yards inland. 

 It therefore has a cross section of 8,750 square yards and contains 

 246,400,000 cubic yards. To this should be added the portion of the 

 beach under water. This, taken for the entire distance from Chicago to 

 Michigan City, is estimated to be about 1,011,890,000 cubic yards. The 

 computation of the subaqueous belt is as follows: The sand at the shore 

 line is about 10 feet deep, and it extends out to where the water reaches a 

 depth of 24 to 36 feet, The breadth varies greatly, ranging from about 

 1,000 yards to nearly 5 miles, the widest part being at the head of the lake. 

 The total bulk of the lake deposits, both in and out of the water, in the 

 section between Michigan City and the mouth of Chicago River, is esti- 

 mated to be 1,747,570,000 cubic yards, or about one-third of a cubic mile. 



Dr. Andrews estimates that the combined bulk of the beaches formed 

 by Lake Chicago is nearly equal to that of the beach of the present lake, 

 the proportion being 16 to 17. In this computation it was assumed that 

 656,000,000 cubic yards, or nearly one-eighth of a cubic mile, escaped 

 through the Chicago Outlet. This assumption is based on .a comparisdn of 

 the relative sizes of the beaches of Lake Chicago in a section outside the 

 outlet and a section embracing the outlet. 



