ces ▁on ▁either ▁side ▁of ▁the ▁present ▁flo od ▁pla ins . ▁ ▁When ▁the ▁ice ▁of ▁the ▁last ▁gla cial ▁epo ch ▁had ▁retre ated ▁so ▁far ▁that ▁its ▁front ▁border ▁lay ▁on ▁a ▁north ward ▁slope , ▁belonging ▁to ▁the ▁d rain age ▁area ▁of ▁the ▁Great ▁L akes , ▁bodies ▁of ▁water ▁accum ulated ▁in ▁front ▁of ▁the ▁ice ▁margin , ▁forming ▁gla cio - m arg inal ▁la kes . ▁The ▁la kes ▁were ▁small ▁at ▁first , ▁and ▁each ▁had ▁its ▁own ▁out let ▁at ▁the ▁lowest ▁de pression ▁of ▁land ▁to ▁the ▁south . ▁As ▁the ▁ice ▁m elt ed ▁further ▁back , ▁neighbor ing ▁la kes ▁became ▁confl uent ▁at ▁the ▁level ▁of ▁the ▁lowest ▁out let ▁of ▁the ▁group . ▁The ▁out flow ing ▁streams ▁grew ▁in ▁the ▁same ▁proportion ▁and ▁er oded ▁a ▁broad ▁channel ▁across ▁the ▁height ▁of ▁land ▁and ▁far ▁down ▁stream , ▁while ▁the ▁lake ▁waters ▁built ▁sand ▁re ef s ▁or ▁car ved ▁shore ▁cl iffs ▁along ▁their ▁margin , ▁and ▁laid ▁down ▁sheets ▁of ▁cl ay ▁on ▁their ▁flo ors . ▁All ▁of ▁these ▁features ▁are ▁easily ▁recognized ▁in ▁the ▁pra irie ▁region . ▁The ▁present ▁site ▁of ▁Chicago ▁was ▁determined ▁by ▁an ▁Indian ▁port age ▁or ▁carry ▁across ▁the ▁low ▁divide ▁between ▁Lake ▁Michigan ▁and ▁the ▁head w aters ▁of ▁the ▁Illinois ▁River . ▁This ▁divide ▁lies ▁on ▁the ▁floor ▁of ▁the ▁former ▁out let ▁channel ▁of ▁the ▁gla cial ▁Lake ▁Michigan . ▁Cor respond ing ▁out lets