▁to ▁be ▁shown ▁on ▁any ▁maps ▁except ▁of ▁the ▁largest ▁scale . ▁Small ▁as ▁they ▁are , ▁they ▁are ▁the ▁chief ▁relief ▁of ▁the ▁pra irie ▁states , ▁and , ▁in ▁association ▁with ▁the ▁nearly ▁imper cept ible ▁s lop es ▁of ▁the ▁till ▁pla ins , ▁they ▁determine ▁the ▁course ▁of ▁many ▁streams ▁and ▁rivers , ▁which ▁as ▁a ▁whole ▁are ▁consequ ent ▁upon ▁the ▁surface ▁form ▁of ▁the ▁gla cial ▁depos its . ▁ ▁The ▁complexity ▁of ▁the ▁gla cial ▁period ▁and ▁its ▁sub div ision ▁into ▁several ▁gla cial ▁epo chs , ▁separated ▁by ▁inter g la cial ▁epo chs ▁of ▁considerable ▁length ▁( certain ly ▁longer ▁than ▁the ▁post g la cial ▁epo ch ) ▁has ▁a ▁struct ural ▁consequence ▁in ▁the ▁super position ▁of ▁success ive ▁till ▁sheets , ▁altern ating ▁with ▁non - g la cial ▁depos its . ▁It ▁also ▁has ▁a ▁phys i ographic ▁consequence ▁in ▁the ▁very ▁different ▁amount ▁of ▁normal ▁post g la cial ▁er os ion ▁suffered ▁by ▁the ▁different ▁parts ▁of ▁the ▁gla cial ▁depos its . ▁The ▁southern most ▁dr ift ▁sheets , ▁as ▁in ▁southern ▁Iowa ▁and ▁northern ▁Missouri , ▁have ▁lost ▁their ▁initially ▁plain ▁surface ▁and ▁are ▁now ▁m ature ly ▁disse cted ▁into ▁grace fully ▁rolling ▁forms . ▁Here , ▁the ▁valle ys ▁of ▁even ▁the ▁small ▁streams ▁are ▁well ▁opened ▁and ▁grad ed , ▁and ▁mar sh es ▁and ▁la kes ▁are ▁rare . ▁These ▁sheets ▁are ▁of ▁early ▁Ple ist oc ene ▁origin . ▁Near