▁London ▁as ▁far ▁as ▁the ▁Ch il tern ▁Hills . ▁ ▁As ▁to ▁to ps o il ▁the ▁river ▁marks ▁a ▁divide ▁between ▁per me able ▁shall ow ▁gra vel ly ▁to ps o il ▁for ▁many ▁miles ▁to ▁the ▁north ▁and ▁almost ▁imper me able ▁all u vi um ▁to ▁the ▁south . ▁ ▁A ▁very ▁thin , ▁agricult ur ally ▁prosper ous ▁all uv ial ▁bel t ▁by ▁the ▁River ▁Th ames ▁ ▁to ▁ ▁away ▁was ▁caused ▁by ▁de position ▁from ▁season al ▁flo ods . ▁This ▁prevents ▁for ▁ 6 ▁miles ▁the ▁rivers ▁joining ▁despite ▁their ▁proxim ity . ▁ ▁As ▁the ▁gra vel ▁former ▁terra ces ▁of ▁the ▁Th ames ▁slope ▁from ▁north ▁to ▁south ▁lower ing ▁the ▁water ▁table ▁as ▁far ▁as ▁the ▁more ▁cl ay - rich ▁parts ▁of ▁the ▁north , ▁the ▁Ash ▁collect s ▁almost ▁all ▁its ▁water ▁from ▁the ▁Col ne ▁having ▁no ▁spr ings ▁of ▁its ▁own , ▁and ▁having ▁a ▁bed ▁largely ▁divor cing ▁it ▁from ▁the ▁rise ▁and ▁fall ▁of ▁the ▁Th ames ▁Bas in ' s ▁water ▁table . ▁ ▁Water ▁quality ▁is ▁good ▁despite ▁the ▁eff lu ent ▁treatment ▁works ▁along ▁the ▁Col ne ▁and ▁for ▁a ▁slow - flow ing ▁river ▁e ut roph ic ation ▁and ▁de position ▁is ▁moder ate ▁to ▁low . ▁ ▁History ▁Ash ford ▁g ains ▁its ▁name ▁from ▁the ▁river , ▁forming ▁the ▁southern ▁boundary ▁of ▁all ▁but ▁its ▁eastern , ▁former ▁common ▁land ▁part . ▁▁ ▁Maps ▁featuring ▁the ▁Ash