▁ice - border ed ▁out let ▁river . ▁Success ive ▁channels ▁are ▁found ▁at ▁lower ▁and ▁lower ▁levels ▁on ▁the ▁plate au ▁slope , ▁indicating ▁the ▁success ive ▁courses ▁taken ▁by ▁the ▁lake ▁out let ▁as ▁the ▁ice ▁m elt ed ▁farther ▁and ▁farther ▁back . ▁On ▁some ▁of ▁these ▁channels , ▁deep ▁g org es ▁were ▁er oded ▁heading ▁in ▁temporary ▁cat ar act s ▁which ▁exceed ed ▁Ni ag ara ▁in ▁height ▁but ▁not ▁in ▁bread th . ▁The ▁po ols ▁exc av ated ▁by ▁the ▁pl ung ing ▁waters ▁at ▁the ▁head ▁of ▁the ▁g org es ▁are ▁now ▁occupied ▁by ▁little ▁la kes . ▁The ▁most ▁significant ▁stage ▁in ▁this ▁series ▁of ▁changes ▁occurred ▁when ▁the ▁gla cio - m arg inal ▁lake ▁waters ▁were ▁lower ed ▁so ▁that ▁the ▁long ▁esc arp ment ▁of ▁Ni ag ara ▁lim estone ▁was ▁laid ▁bare ▁in ▁western ▁New ▁York . ▁The ▁previously ▁confl uent ▁waters ▁were ▁then ▁divided ▁into ▁two ▁la kes . ▁The ▁higher ▁one , ▁Lake ▁Er ie , ▁supplied ▁the ▁out flow ing ▁Ni ag ara ▁River , ▁which ▁pou red ▁its ▁waters ▁down ▁the ▁esc arp ment ▁to ▁the ▁lower , ▁Lake ▁Ontario . ▁This ▁gave ▁rise ▁to ▁Ni ag ara ▁F alls . ▁Lake ▁Ontario ' s ▁out let ▁for ▁a ▁time ▁ran ▁down ▁the ▁Moh awk ▁Valley ▁to ▁the ▁Hudson ▁River . ▁At ▁this ▁higher ▁elev ation , ▁it ▁was ▁known ▁as ▁Lake ▁I ro qu ois . ▁When ▁the ▁ice ▁m elt ed ▁from