la cial ▁origin . ▁It ▁is ▁thus ▁by ▁sub - g la cial ▁ag grad ation ▁that ▁the ▁pra i ries ▁have ▁been ▁level led ▁up ▁to ▁a ▁smooth ▁surface , ▁in ▁contrast ▁to ▁the ▁higher ▁and ▁non - g la ci ated ▁h illy ▁country ▁just ▁to ▁the ▁south . ▁ ▁The ▁great ▁ice ▁sheets ▁formed ▁terminal ▁mor aines ▁around ▁their ▁border ▁at ▁various ▁end ▁stages . ▁However , ▁the ▁mor ain ic ▁bel ts ▁are ▁of ▁small ▁relief ▁in ▁comparison ▁to ▁the ▁great ▁area ▁of ▁the ▁ice . ▁They ▁rise ▁g ently ▁from ▁the ▁till ▁pla ins ▁to ▁a ▁height ▁of ▁ 5 0 , ▁ 1 0 0 ▁or ▁more ▁feet . ▁They ▁may ▁be ▁one , ▁two ▁or ▁three ▁miles ▁( 5   km ) ▁wide ▁and ▁their ▁h illy ▁surface , ▁d otted ▁over ▁with ▁b ould ers , ▁contains ▁many ▁small ▁la kes ▁in ▁bas ins ▁or ▁h ollow s , ▁instead ▁of ▁streams ▁in ▁valle ys . ▁The ▁mor ain ic ▁bel ts ▁are ▁arranged ▁in ▁groups ▁of ▁concent ric ▁loops , ▁convex ▁south ward , ▁because ▁the ▁ice ▁sheets ▁advanced ▁in ▁lo bes ▁along ▁the ▁low lands ▁of ▁the ▁Great ▁L akes . ▁Ne igh bor ing ▁mor ain ic ▁loops ▁join ▁each ▁other ▁in ▁re - entr ants ▁( n orth - point ing ▁c us ps ), ▁where ▁two ▁adjacent ▁gla cial ▁lo bes ▁came ▁together ▁and ▁formed ▁their ▁mor aines ▁in ▁largest ▁volume . ▁The ▁mor aines ▁are ▁of ▁too ▁small ▁relief