▁the ▁n ort he astern ▁end ▁of ▁the ▁lake , ▁it ▁dropped ▁to ▁a ▁lower ▁level , ▁and ▁dra ined ▁through ▁the ▁St . ▁Lawrence ▁area . ▁This ▁created ▁a ▁lower ▁base ▁level ▁for ▁the ▁Ni ag ara ▁River , ▁increasing ▁its ▁er os ive ▁capacity . ▁ ▁In ▁certain ▁districts , ▁the ▁sub g la cial ▁till ▁was ▁not ▁spread ▁out ▁in ▁a ▁smooth ▁plain , ▁but ▁accum ulated ▁in ▁elli pt ical ▁m ounds , ▁ 1 0 0 – 2 0 0 ▁feet . ▁high ▁and ▁ ▁long ▁with ▁axes ▁parallel ▁to ▁the ▁direction ▁of ▁the ▁ice ▁motion ▁as ▁indicated ▁by ▁st ria e ▁on ▁the ▁underlying ▁rock ▁floor . ▁These ▁hills ▁are ▁known ▁by ▁the ▁Irish ▁name , ▁drum l ins , ▁used ▁for ▁similar ▁hills ▁in ▁north - western ▁Ireland . ▁The ▁most ▁remarkable ▁groups ▁of ▁drum l ins ▁occur ▁in ▁western ▁New ▁York , ▁where ▁their ▁number ▁is ▁estimated ▁at ▁over ▁ 6 , 0 0 0 , ▁and ▁in ▁southern ▁Wisconsin , ▁where ▁it ▁is ▁placed ▁at ▁ 5 , 0 0 0 . ▁They ▁completely ▁domin ate ▁the ▁top ography ▁of ▁their ▁districts . ▁ ▁A ▁curious ▁depos it ▁of ▁an ▁imp al p ably ▁fine ▁and ▁un str at ified ▁s ilt , ▁known ▁by ▁the ▁German ▁name ▁b ess ▁( or ▁lo ess ), ▁lies ▁on ▁the ▁older ▁dr ift ▁sheets ▁near ▁the ▁larger ▁river ▁courses ▁of ▁the ▁upper ▁Mississippi ▁bas in . ▁It ▁att ains ▁a ▁thick ness ▁of ▁ ▁or ▁more