▁near ▁the ▁rivers ▁and ▁gradually ▁f ades ▁away ▁at ▁a ▁distance ▁of ▁ten ▁or ▁more ▁miles ▁( 1 6 ▁or ▁more ▁km ) ▁on ▁either ▁side . ▁It ▁contains ▁land ▁shell s , ▁and ▁hence ▁cannot ▁be ▁attributed ▁to ▁marine ▁or ▁lac ustr ine ▁sub mer gence . ▁The ▁best ▁explanation ▁is ▁that , ▁during ▁certain ▁phases ▁of ▁the ▁gla cial ▁period , ▁it ▁was ▁carried ▁as ▁dust ▁by ▁the ▁wind s ▁from ▁the ▁flo od ▁pla ins ▁of ▁ag grad ing ▁rivers , ▁and ▁slowly ▁depos ited ▁on ▁the ▁neighbor ing ▁grass - cover ed ▁pla ins . ▁The ▁gla cial ▁and ▁e ol ian ▁origin ▁of ▁this ▁sed iment ▁is ▁ev iden ced ▁by ▁the ▁angular ity ▁of ▁its ▁gra ins ▁( a ▁bank ▁of ▁it ▁will ▁stand ▁without ▁sl ump ing ▁for ▁years ), ▁whereas , ▁if ▁it ▁had ▁been ▁transport ed ▁significantly ▁by ▁water , ▁the ▁gra ins ▁would ▁have ▁been ▁rounded ▁and ▁pol ished . ▁Lo ess ▁is ▁parent ▁material ▁for ▁an ▁extremely ▁fert ile , ▁but ▁dr ought y ▁soil . ▁ ▁South western ▁Wisconsin ▁and ▁parts ▁of ▁the ▁adjacent ▁states ▁of ▁Illinois , ▁Iowa , ▁and ▁Minnesota ▁are ▁known ▁as ▁the ▁dr ift less ▁zone , ▁because , ▁although ▁border ed ▁by ▁dr ift ▁sheets ▁and ▁mor aines , ▁it ▁is ▁free ▁from ▁gla cial ▁depos its . ▁It ▁must ▁therefore ▁have ▁been ▁a ▁sort ▁of ▁o asis , ▁when ▁the ▁ice ▁sheets ▁from ▁the ▁north ▁advanced ▁past ▁it ▁on ▁the ▁east ▁and ▁west , ▁and