CHANGES IN THE CHICAGO OUTLET 



243 



foot level in the Lake Erie Basin, would have to find its way through the 

 Cliicago outlet. No one who examines the facts can hesitate to believe 

 in tlie sufficiency of this water supply to produce the known results. It 

 is fair to say that this explanation is suggested in a single sentence by 

 Mr. F. B. Taylor on page 326 of Monograph LIII, where he says: ^^The 

 changes which occurred in Lake Chicago were due to erosion of its outlet 

 or to changes in the volume of its discharge." 



Glacial and Clay Deposits underneath Chicago 



Mr. Shedd calls attention to a deposit of soft putty-like blue clay that 

 overlies the hard-pan glacial drift underneath all the Chicago region, 

 wherever caissons have been sunk or channels dug. From information 

 furnished by Mr. George W. Jackson, who has built 110 miles of tunnel 

 under the city of Chicago, and 

 from Mr. John Griffiths and other 

 engineers who have sunk innu- 

 merable caissons to the solid rock 

 for the support of the great build- 

 ings of the city, we learn that be- 

 low a few feet of muck and yellow 

 loam, which form the surface de- 

 posit over this area, there is regu- 

 larly from 20 to 40 feet of soft 

 blue clay resting on the till, which 

 extends in most cases about 60 

 feet farther down to the solid 

 rock, which in many cases shows 

 grooves and stretches character- 

 istic of glacial surfaces. Mr. 

 Griffiths gave me a piece of Lake 

 Superior copper which he had 

 found a few days before, 60 feet 

 below the surface, in a caisson 

 which he was sinking at the cor- 

 ner of Harrison and Canal streets. 

 Mr. Jackson reports that beneath 

 there frequently occurs a stratum 

 gravel from the size of a pinhead 



Figure .3. — Section of laminated Clay abdre 

 the soft Wue Clay appearing in the diver- 

 f<inii Channel at Eranston. 



the till over the hard limestone rock 



3 or 4 inches thick of "black torpedo 



to a ])ea.'' This succession of deposits 



continues substantially the same 2 or 



rfackson Park, where he put in the Hyde 



crib. Mr. Shedd also reports a similar succession 



miles out into the lake east of 



Park water works tunnel and 



it Wolf Lake, in tlie 



