ift . ▁ ▁The ▁greatest ▁area ▁of ▁the ▁pra i ries , ▁from ▁Indiana ▁to ▁North ▁Dak ota , ▁consists ▁of ▁till ▁pla ins , ▁that ▁is , ▁sheets ▁of ▁un str at ified ▁dr ift . ▁These ▁pla ins ▁are ▁ 3 0 , ▁ 5 0 ▁or ▁even ▁ 1 0 0   ft ▁( up ▁to ▁ 3 0   m ) ▁thick ▁covering ▁the ▁underlying ▁rock ▁surface ▁for ▁thousands ▁of ▁square ▁miles ▁except ▁where ▁post g la cial ▁stream ▁er os ion ▁has ▁locally ▁laid ▁it ▁bare . ▁The ▁pla ins ▁have ▁an ▁extra ordin arily ▁even ▁surface . ▁The ▁till ▁is ▁pres umably ▁made ▁in ▁part ▁of ▁preg la cial ▁so ils , ▁but ▁it ▁is ▁more ▁largely ▁composed ▁of ▁rock ▁waste ▁mechan ically ▁transport ed ▁by ▁the ▁cre eping ▁ice ▁sheets . ▁Although ▁the ▁cry st all ine ▁rocks ▁from ▁Canada ▁and ▁some ▁of ▁the ▁more ▁resist ant ▁strat ified ▁rocks ▁south ▁of ▁the ▁Great ▁L akes ▁occur ▁as ▁b ould ers ▁and ▁stones , ▁a ▁great ▁part ▁of ▁the ▁till ▁has ▁been ▁cr ushed ▁and ▁ground ▁to ▁a ▁cl ay ey ▁texture . ▁The ▁till ▁pla ins , ▁although ▁swe eping ▁in ▁broad ▁sw ells ▁of ▁slowly ▁changing ▁alt itude , ▁often ▁appear ▁level ▁to ▁the ▁eye ▁with ▁a ▁view ▁stretch ing ▁to ▁the ▁horizon . ▁Here ▁and ▁there , ▁faint ▁de press ions ▁occur , ▁occupied ▁by ▁mar sh y ▁sl ough s , ▁or ▁flo ored ▁with ▁a ▁rich ▁black ▁soil ▁of ▁post g