▁Hills ▁themselves , ▁it ▁is ▁thought ▁that ▁the ▁er upt ive ▁cent res ▁were ▁south ▁of ▁the ▁fault , ▁however ▁these ▁are ▁buried ▁deep ▁beneath ▁the ▁Car bon ifer ous ▁rocks ▁including ▁coal ▁measures ▁which ▁are ▁found ▁beneath ▁the ▁low - lying ▁car se ▁lands ▁to ▁the ▁south . ▁ ▁To ▁put ▁this ▁in ▁perspective , ▁the ▁prominent ▁hill ▁Dum y at ▁which ▁defines ▁the ▁western ▁extrem ity ▁of ▁the ▁O ch il ▁Hills ▁has ▁a ▁height ▁of ▁ 4 1 8 ▁metres . ▁The ▁nearest ▁col li ery , ▁Man or ▁Pow is , ▁approximately ▁ 2   km ▁south ▁of ▁the ▁fault , ▁was ▁approximately ▁the ▁same ▁depth , ▁in ▁car se ▁land ▁that ▁is ▁scarcely ▁above ▁sea ▁level , ▁and ▁was ▁min ing ▁coal ▁from ▁Car bon ifer ous ▁str ata ▁at ▁least ▁many ▁hundreds ▁of ▁metres ▁above ▁the ▁top ▁of ▁the ▁Dev onian ▁depos its . ▁The ▁O ch il ▁F ault ▁was ▁active ▁from ▁the ▁earliest ▁part ▁of ▁the ▁Car bon ifer ous ▁period , ▁allowing ▁the ▁marine ▁sw amps ▁in ▁which ▁the ▁coal - produ cing ▁veget ation ▁fl our ished ▁to ▁be ▁period ically ▁over lain ▁by ▁layers ▁of ▁sand , ▁forming ▁sand stone , ▁or ▁s ilt , ▁forming ▁sh ale , ▁in ▁a ▁number ▁of ▁cy clic ▁subs id ences . ▁These ▁occurred ▁in ▁the ▁form ▁of ▁bas ins , ▁and ▁the ▁pattern ▁of ▁movements ▁was ▁complex , ▁however ▁it ▁is ▁known ▁that ▁the ▁greatest ▁vertical ▁movement ▁was ▁cent red ▁about ▁ 1  