▁in ▁the ▁fact ▁that ▁the ▁latter ▁out let ▁was ▁through ▁easily ▁er oded ▁dr ift ▁depos its ▁and ▁the ▁Chicago ▁out let ▁was ▁over ▁a ▁rock ▁s ill . ▁ ▁Lower ▁be aches ▁N umer ous ▁rid ges ▁in ▁the ▁southern ▁portion ▁of ▁the ▁Lake ▁Michigan ▁bas in ▁at ▁levels ▁a ▁little ▁below ▁that ▁of ▁the ▁Tol eston ▁beach ▁probably ▁belong , ▁the ▁higher ▁to ▁Lake ▁Alg on quin ▁and ▁the ▁lower ▁to ▁Lake ▁N ip iss ing . ▁Most ▁of ▁them ▁rise ▁ ▁above ▁Lake ▁Michigan , ▁or ▁about ▁ ▁above ▁sea ▁level , ▁but ▁some ▁reach ▁about ▁ ▁and ▁others ▁not ▁more ▁than ▁. ▁The ▁Chicago ▁out let ▁seems ▁to ▁have ▁ceased ▁to ▁be ▁functional ▁at ▁the ▁time ▁they ▁were ▁developed , ▁for ▁they ▁in ▁a ▁measure ▁cho ke ▁up ▁or ▁bridge ▁over ▁its ▁head . ▁Furthermore , ▁they ▁are ▁about ▁as ▁low ▁as ▁the ▁bed ▁of ▁the ▁out let , ▁so ▁that ▁no ▁depth ▁of ▁water ▁would ▁have ▁been ▁possible ▁along ▁the ▁out let . ▁These ▁rid ges ▁are ▁exception ally ▁well ▁displayed ▁in ▁the ▁south ▁part ▁of ▁Chicago ▁in ▁the ▁vic inity ▁of ▁the ▁university , ▁and ▁they ▁are ▁even ▁better ▁developed ▁just ▁east ▁of ▁the ▁Illinois - Ind iana ▁State ▁line . ▁On ▁a ▁line ▁running ▁north ▁from ▁Gib son ▁station ▁there ▁are ▁by ▁actual ▁count ▁ 3 2   be ach lets ▁separated ▁by ▁shall ow ▁s ags . ▁North ▁of ▁W au k eg on ▁a ▁cut ▁bank ▁appears ▁at ▁a ▁level ▁corresponding ▁to ▁that ▁of