78 CHICAGO ACADEMY OF SCIENCES. 



These do not form continuous lines around the head of the lake, 

 but those in the vicinity of the Chicago University and Jackson 

 Park die out in the marsh, which sets in a short distance south of 

 the park, and those in Lake County, Indiana, die out at their 

 western ends in a sandy plain, which borders Wolf Lake, Lake 

 Calumet, and other small lakes near the State line. This sandy 

 plain stands but 5 to 8 feet above the lake and was apparently an 

 open bay at the time these bar-like features were forming. But it 

 has now become filled with sand, leaving Lake Calumet and the 

 other small lakes as its dwarfed representatives. The beachlets 

 stand only lo to 12 feet above lake level (except where coated by 

 wind-drifted sand) and are, as noted above, to be referred to the 

 action of the present lake rather than to Lake Chicago. 



The outlets of the lake at the time the Third beach was form- 

 ing appear to have been along three lines; one, that occupied by 

 the mouth of the Chicago River and the south branch of the Chi- 

 cago River (reversed) ; a second along the marsh referred to above 

 as leading from the south part of Hyde Park Township north- 

 westward between Englewood and South Lynne, which connects 

 with the south fork of the Chicago River north of the Union Stock 

 Yards ; a third leading westward from Riverdale along the Sag out- 

 let. The broadest of these outlets is that leading past Englewood 

 and the Union Stock Yards, and it is possible that the other out- 

 lets became nearly closed by sand before this outlet was aban- 

 doned. 



The altitude of this beach is nowhere more than 20 to 22 feet 

 above the lake (except where wind has drifted sand to higher 

 levels). The outlets could not well have been cut below a level 8 

 feet above the lake, that being the altitude of the Chicago outlet for 

 several miles below its junction with the present Des Plaines River. 

 The depth of the water in the outlets would, therefore, be 10 to 12 

 feet or less. As beaches are often built up to a height of 4 or 5 feet 

 above the ordinary level of the lake, it seems probable that 

 the ordinary stage of water was not more than 15 feet above the 

 present stage of Lake Michigan, thus leaving but 7 feet depth 

 of water in the outlet. The Sag outlet reaches nearly 15 feet 

 above the level of Lake Michigan, hence it was probably at all 

 times a minor line of discharge. 



As hinted above, the reference of this beach to the same lake 

 which formed the higher beaches is not made with any degree of 

 confidence. Indeed, the abundant life of the waters which formed 



