ap . ▁This ▁section ▁of ▁the ▁valley ▁is ▁now ▁blocked ▁by ▁a ▁filling ▁of ▁gla cial ▁till , ▁diver ting ▁the ▁river ▁south ▁west , ▁but ▁the ▁old ▁course ▁has ▁been ▁detected ▁by ▁dr illing ▁and ▁echo - s ounding . ▁The ▁rapidly ▁er oding ▁cl iffs ▁threaten ing ▁the ▁southern ▁edge ▁of ▁Bird os wald ▁Roman ▁Fort ▁and ▁the ▁fact ▁that ▁the ▁Roman ▁bridge ▁ab ut ment ▁at ▁Will ow ford ▁is ▁now ▁ 4 4   m ▁( 1 4 4 . 4   ft ) ▁from ▁the ▁river ▁test ify ▁to ▁the ▁inst ability ▁of ▁the ▁local ▁landscape ▁as ▁it ▁continues ▁to ▁recover ▁from ▁gla cial ▁alter ation . ▁ ▁The ▁river ▁r ises ▁on ▁P add ab urn ▁Mo or ▁in ▁K iel der ▁Forest ▁and ▁sk ir ts ▁the ▁eastern ▁edge ▁of ▁But ter burn ▁Flow ▁raised ▁bog . ▁Rem ains ▁of ▁numerous ▁media eval ▁sh iel ings ▁have ▁been ▁found ▁along ▁the ▁banks ▁of ▁these ▁upper ▁reaches , ▁some ▁of ▁which ▁have ▁been ▁given ▁ancient ▁monument ▁status . ▁ ▁For ▁two ▁miles ▁above ▁the ▁village ▁of ▁G ils land ▁the ▁river ▁flows ▁through ▁the ▁Ir thing ▁G orge ▁, ▁fl ank ed ▁by ▁mainly ▁sand stone ▁c rag s . ▁Within ▁the ▁upper ▁section ▁of ▁this ▁g orge ▁lies ▁the ▁spect ac ular ▁ 7 . 6 ▁met re ▁( 2 5 ▁feet ) ▁C ram mel ▁L inn ▁water fall . ▁Per egr ine ▁fal cons ▁and ▁ra vens ▁nest ▁near ▁the ▁water fall ▁and ▁der