When the ice of the last glacial epoch had retreated so far that its front border lay on a northward slope, belonging to the drainage area of the Great Lakes, bodies of water accumulated in front of the ice margin, forming glacio-marginal lakes. The lakes were small at first, and each had its own outlet at the lowest depression of land to the south. As the ice melted further back, neighboring lakes became confluent at the level of the lowest outlet of the group. The outflowing streams grew in the same proportion and eroded a broad channel across the height of land and far down stream, while the lake waters built sand reefs or carved shore cliffs along their margin, and laid down sheets of clay on their floors. All of these features are easily recognized in the prairie region. The present site of Chicago was determined by an Indian portage or carry across the low divide between Lake Michigan and the headwaters of the Illinois River. This divide lies on the floor of the former outlet channel of the glacial Lake Michigan. Corresponding outlets are known for Lake Erie, Lake Huron, and Lake Superior. A very large sheet of water, named Lake Agassiz, once overspread a broad till plain in northern Minnesota and North Dakota. The outlet of this glacial lake, called river Warren, eroded a large channel in which the Minnesota River evident today. The Red River of the North flows northward through a plain formerly covered by Lake Agassiz.
when the iCe OF The last glacial ePoch hAd reTreATed sO fAR thAT its froNt boRder lAY ON a nOrtHWard slope, BelonginG to THE DrAinage AreA of ThE gReAt lakEs, bodies of WatER acCumulaTed iN FroNt oF the icE MaRgIn, FoRmIng GLacIo-MaRgINaL Lakes. the lAkeS weRE SmalL at firsT, ANd each HAd iTs oWN ouTLet at tHe loweSt dEprEsSioN Of laND to the SoUth. as tHE ice melTed FuRther baCK, neiGhboriNg lakES bEcAme cOnflUENt at the LeVel oF tHe lOWEsT oUTlEt oF tHE grOUp. THe outfLoWinG STReaMs gRew In THe same ProPortioN and eroDeD A broAd chANnel AcrosS the HeighT Of LAnd And FaR dowN StrEam, wHilE thE LakE WAtErs bUilt saNd reefs oR CarvEd shore cLIffs along tHeIR MARGiN, ANd laid dOWn SheeTs of clAY on tHeir fLoORS. all oF These FeaTures aRe easIlY reCoGnized IN the prAiriE RegIon. thE present siTe of chicAgo was DeteRmIned By an indian porTage or carry Across tHE loW Divide betweEN lAke michiGaN and the headwATers of thE IllInoiS rivEr. THis Divide lIes on ThE FlOoR of tHE fORmer outlet chAnNEl of THe glaCial Lake MichIGAN. coRrEspOnding ouTletS arE KnOWN for Lake erIE, laKe huron, And LAke SuPerIor. a vERY large sHeet of waTer, NameD lake agaSSiz, once OVerSPREad a broad till PLaIn iN Northern miNnesota anD nORth dAKotA. the oUTlet of tHis glAcIal lAkE, CalLed RiVEr warRen, ErODeD A LargE chAnnEL iN wHiCh The minNeSOTa RivEr eViDeNt today. thE rED river Of the north fLows nORthward tHRough a plaiN FormeRLY CoverED BY laKe AGASsIz.