Lake Chicago
Map of middle stage of glacial Lake Chicago, USGS Report of 1915.
Lake Chicago
Location	North America
Group	Great Lakes
Coordinates	42°12′N87°06′W / 42.2°N 87.1°W /42.2; -87.1
Lake type	former lake
Primary inflows	Laurentide Ice Sheet
Primary outflows	Chicago River
Basin countries	United States
First flooded	18,000 years before present
Max. length	241 mi (388 km)
Max. width	57 mi (92 km)
Average depth	160 ft (49 m)[1]
Residence time	4000 years in existence
Surface elevation	160 ft (49 m)
References	The Pleistocene of Indiana and Michigan and the History of the Great Lakes; Frank Leverett and Frank B. Taylor; Monographs of the United States Geological Survey; Volume LIII; Washington; 1915

Lake Chicago was a prehistoricproglacial lake that is the ancestor of what is now known asLake Michigan, one ofNorth America's fiveGreat Lakes. Formed about 13,000 years ago and fed by retreating glaciers, it drained southwest through the Chicago Outlet River.

The city ofChicago lies in a broad plain that, hundreds of millions of years ago, was part of a great interior basin covered by warm, shallow seas. These seas covered portions of North America from theArctic Ocean to theGulf of Mexico. Evidence of these seas is found in thefossils ofcoral, such as those unearthed inIllinoisquarries atStony Island Avenue,Thornton Quarry, andMcCook, Illinois, or at 18th Street and Damen Avenue in Chicago. Evidence may also be found in the fossils in the Niagara limestone bedrock found throughout the Chicago area and extending all the way toNiagara, New York.^[2]

Much later, thepolar ice cap crept four times down across the continent, covering the region with ice to a depth of a mile (1500 m) or more. As the climate changed, the ice melted, the last great ice flow (theWisconsin Glacier of thePleistocene period, which covered much of the northern half of North America) retreated, and an outlet for the melting water developed through the Sag River and theDes Plaines River Valley aroundMt. Forest, in the area known as the Palos. TheKankakee Torrent poured through those valleys, eventually leaving behind the prehistoric Lake Chicago, the ancestor of Lake Michigan.^[2]

13,000 to 11,000 years before present

The earliest formation of Lake Chicago occurred when the Michigan Lobe of the glacier retreated northward into the basin of modernLake Michigan, ca 13,000 years ago.^[3] The edge of the retreating glacier formedmoraines, thePark Moraine in present-day Illinois and theLake Borders Moraine in Indiana and Michigan.^[4]

The ice continued northward forming the next phase of Lake Chicago. The lake level remained at 640' above sea level,^[3] but the ice margin was a third of the way north, opening a channel acrossMichigan, draining theLake Saginaw andLake Whittleseyproglacial lakes in theLake Huron andLake Erie basins.^[4]

When the ice retreated northward enough to open theMohawk River valley, the waters in the Lake Huron and Lake Erie basin diverted from the Michigan outlet, leaving Lake Chicago as a headwaters lake. Continued northward migration of the glacier set the stage for the next major proglacial lakes in the Lake Michigan basin asLake Algonquin and thenNipissing Great Lakes.^[4]

Extending somewhat further south, west and east than Lake Michigan, Lake Chicago extended west to present dayLa Grange, Illinois and south beyondHomewood andLansing, Illinois, completely covering what is nowNorthwest Indiana, including the cities ofHammond andGary, Indiana.

As theWisconsin Glacier continued to retreat, it created new outlets for the water in Lake Chicago, includingNiagara Falls, and theSt. Lawrence River. As these outlets were developed, a partiallake capture occurred and the water level in Lake Chicago began to drop in three observable stages of 15 to 20 feet (5–6 m) each. Eventually even the outlet to the southwest dried up, and the Des Plaines River overflowed into the basin that became Lake Michigan.

As the Michigan Lobe of theLabradorean Ice Cap retreated northward, the area of Lake Chicago increased.^[5] Lake Chicago’s initial outlet was to the southwest through theDes Plaines River valley, thence down theIllinois River to theGulf of Mexico. The highest beach is theGlenwood Shoreline, which is 55 feet (17 m) to 60 feet (18 m) above the level of Lake Michigan. There are two additional beaches formed when the Des Plaines outlet was abandoned for other outlets to the north and east: theCalumet Shoreline, about 35 feet (11 m) to 40 feet (12 m) above the current lake, and theTolleston Beach 20 feet (6.1 m) to 25 feet (7.6 m) above Lake Michigan. The name Lake Chicago is used for the lake when it drains through the Des Plaines outlet. The Calumet and Tolleston Beach lake isLake Algonquin.^[5]

The outlet channel is more than 1 mile (1.6 km) wide and cuts through glacial deposits. Near the head of the channel, rock layers have been cut through. It is probable that the distinct beaches from Lake Chicago are the result of these rock layers giving way rapidly, dropping the lake level from higher to lower levels. After each stage, the next barrier remained solid, holding the lake stable and creating distinct beaches. If the outlet were formed by a steady erosion of the barrier, it would have been less likely that the well-defined beaches would have been created.^[5]

Along the eastern shore of Lake Michigan, the beaches of Lake Chicago have been destroyed by erosion, except the highest beach, much of which has also been destroyed.^[5] The best remaining segments are along the southern tip of Lake Michigan in Indiana.

Lake Chicago covered only a narrow strip of land on the south and east side of modern Lake Michigan. FromHolland northward across the Lake Chicago, the lake bed extends 10 to 25 miles (16 to 40 km) inland. The widest expanse is almost entirely fine sand. There is more sand than clay in a narrow strip along the shore. The clayey portions have till at only a few inches depth. The offshore winds along the east shore of Lake Michigan have created sand dunes, burying the older glacial beaches and lake beds.^[5]

Vast amounts of sand inspits,dunes and beach lines—particularly at the southern tip of Lake Michigan—were left behind by each of the three stages of lake level drop. Today, evidence of these vast sand deposits is still clearly visible. Northern Indiana, for example, contains theIndiana Dunes, and many of the Chicago area's trails and roads follow some of these ancient beach lines or ridges in the sand spits.

For example, Ridge Road from Homewood, Illinois, through Thornton andLansing, Illinois, and then crossing the state border into Munster andHighland, Indiana, is one; Michigan City Road through Riverdale, Dolton, and Calumet City, Illinois, is another; LaGrange Road is another; Riverside Drive in Riverside; Ridgeland Avenue in Oak Park, or Grosse Point Road, Carpenter Road, and Ridge Avenue (The Rosehill Spit) in the Rogers Park/West Ridge neighborhood of Chicago, north of Devon Avenue and continuing north through Evanston, Illinois, are some others.Blue Island, Illinois, andStony Island were, literally, islands left behind as Lake Chicago's water level fell.

• Bretz, J.H. 1939. Geology of the Chicago Region, Part 1 - General. Illinois State Geological Survey, Bulletin 65. 118 pages.
• Killey, Myrna M. 1998. Illinois' Ice Age Legacy. Illinois State Geological Survey GeoScience Education Series 14. 66 pages.
• Willman, W.B. 1971. Summary of the Geology of the Chicago Area. Illinois State Geological Survey, Urbana, IL. Circular 460. 77 pages.
• Oak Park - Ancient Lake Chicago & Continental Divide

• Former lakes of the United States
• Geology of Illinois
• Geology of Indiana
• Geological history of the Great Lakes
• Proglacial lakes
• Lake Michigan
• Glacial lakes of the United States
• Geography of Chicago

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