voir ; ▁this ▁had ▁the ▁added ▁advantage ▁that ▁it ▁increased ▁its ▁event ual ▁capacity . ▁Ne ither ▁cl ay ▁nor ▁s ilt ▁could ▁be ▁used ▁as ▁gr out ing ▁material ▁because ▁they ▁did ▁not ▁occur ▁in ▁sufficient ▁quantities ▁in ▁the ▁Upper ▁Har z . ▁The ▁Upper ▁Har z ▁min ers ▁had , ▁however , ▁learned ▁from ▁experience ▁that ▁grass ▁s ods ▁or ▁tur ves ▁made ▁excellent ▁gr out ing ; ▁by ▁placing ▁them ▁one ▁on ▁top ▁of ▁another , ▁in ▁a ▁similar ▁fashion ▁to ▁building ▁a ▁wall , ▁a ▁layer ▁of ▁s ods ▁up ▁to ▁a ▁met re ▁thick ▁would ▁be ▁built ▁into ▁the ▁dam ▁which ▁ens ured ▁that ▁it ▁remained ▁wat ert ight . ▁ ▁A ▁wooden ▁condu it ▁( Hol z ger enne ) ▁acted ▁as ▁a ▁bottom ▁out let . ▁It ▁was ▁usually ▁made ▁of ▁o ak ▁due ▁to ▁its ▁great ▁dur ability . ▁The ▁val ve ▁was ▁a ▁so - called ▁ ▁Str iegel , ▁which ▁acted ▁like ▁a ▁plug ▁and ▁closed ▁off ▁the ▁in let ▁to ▁the ▁wooden ▁channels ▁and ▁was ▁operated ▁by ▁ro ds . ▁Both ▁grass ▁sod ▁gr out ing ▁and ▁wooden ▁bottom ▁out lets ▁are ▁still ▁in ▁use ▁in ▁many ▁of ▁the ▁p onds . ▁ ▁The ▁d ams ▁themselves ▁are ▁between ▁ 4 ▁and ▁ 1 5   m ▁high ▁and ▁the ▁imp ounded ▁volumes ▁varied ▁between ▁ 1 0 , 0 0 0 ▁and ▁ 6 0 0 , 0 0 0   m ³ . ▁One ▁notable ▁exception ▁is ▁the ▁O der