er ▁the ▁Great ▁L akes , ▁the ▁till ▁sheets ▁are ▁tren ched ▁only ▁by ▁the ▁narrow ▁valle ys ▁of ▁the ▁large ▁streams . ▁Marsh y ▁sl ough s ▁still ▁occup y ▁the ▁faint ▁de press ions ▁in ▁the ▁till ▁pla ins ▁and ▁the ▁associated ▁mor aines ▁have ▁abund ant ▁small ▁la kes ▁in ▁their ▁und rained ▁h ollow s . ▁These ▁dr ift ▁sheets ▁are ▁of ▁late ▁Ple ist oc ene ▁origin . ▁ ▁When ▁the ▁ice ▁sheets ▁extended ▁to ▁the ▁land ▁s lop ing ▁south ward ▁to ▁the ▁Ohio ▁River , ▁Mississippi ▁River ▁and ▁Missouri ▁River , ▁the ▁dr ift - lad en ▁streams ▁flow ed ▁freely ▁away ▁from ▁the ▁ice ▁border . ▁As ▁the ▁streams ▁escaped ▁from ▁their ▁sub g la cial ▁channels , ▁they ▁spread ▁into ▁bro ader ▁channels ▁and ▁depos ited ▁some ▁of ▁their ▁load , ▁and ▁thus ▁ag grad ed ▁their ▁courses . ▁Local ▁sheets ▁or ▁apr ons ▁of ▁gra vel ▁and ▁sand ▁are ▁spread ▁more ▁or ▁less ▁abund antly ▁along ▁the ▁outer ▁side ▁of ▁the ▁mor ain ic ▁bel ts . ▁Long ▁trains ▁of ▁gra vel ▁and ▁s ands ▁c log ▁the ▁valle ys ▁that ▁lead ▁south ward ▁from ▁the ▁gla ci ated ▁to ▁the ▁non - g la ci ated ▁area . ▁Later , ▁when ▁the ▁ice ▁retre ated ▁farther ▁and ▁the ▁un loaded ▁streams ▁returned ▁to ▁their ▁earlier ▁de grad ing ▁habit , ▁they ▁more ▁or ▁less ▁completely ▁sc oured ▁out ▁the ▁valley ▁depos its , ▁the ▁remains ▁of ▁which ▁are ▁now ▁seen ▁in ▁terra