▁joined ▁around ▁its ▁southern ▁border . ▁The ▁reason ▁for ▁this ▁exem ption ▁from ▁gla ci ation ▁is ▁the ▁con verse ▁of ▁that ▁for ▁the ▁south ward ▁convex ity ▁of ▁the ▁mor ain ic ▁loops . ▁For ▁while ▁they ▁mark ▁the ▁paths ▁of ▁greatest ▁gla cial ▁advance ▁along ▁low land ▁tr ough s ▁( la ke ▁bas ins ), ▁the ▁dr ift less ▁zone ▁is ▁a ▁district ▁protected ▁from ▁ice ▁invasion ▁by ▁reason ▁of ▁the ▁obst ruction ▁which ▁the ▁high lands ▁of ▁northern ▁Wisconsin ▁and ▁Michigan ▁( part ▁of ▁the ▁Super ior ▁u pl and ) ▁offered ▁to ▁gla cial ▁advance . ▁ ▁The ▁course ▁of ▁the ▁upper ▁Mississippi ▁River ▁is ▁largely ▁consequ ent ▁upon ▁gla cial ▁depos its . ▁Its ▁sources ▁are ▁in ▁the ▁mor ain ic ▁la kes ▁in ▁northern ▁Minnesota . ▁The ▁dr ift ▁depos its ▁there about s ▁are ▁so ▁heavy ▁that ▁the ▁present ▁div ides ▁between ▁the ▁d rain age ▁bas ins ▁of ▁Hudson ▁Bay , ▁Lake ▁Super ior , ▁and ▁the ▁G ulf ▁of ▁Mexico ▁evidently ▁stand ▁in ▁no ▁very ▁definite ▁relation ▁to ▁the ▁preg la cial ▁div ides . ▁The ▁course ▁of ▁the ▁Mississippi ▁through ▁Minnesota ▁is ▁largely ▁gu ided ▁by ▁the ▁form ▁of ▁the ▁dr ift ▁cover . ▁Several ▁rap ids ▁and ▁the ▁Saint ▁Anthony ▁F alls ▁( det erm ining ▁the ▁site ▁of ▁Min ne apolis ) ▁are ▁signs ▁of ▁imm atur ity , ▁resulting ▁from ▁super position ▁through ▁the ▁dr ift ▁on ▁the ▁under ▁rock . ▁Far ther ▁south , ▁as ▁far ▁as ▁the