The High Weald lies at the core of the distinctive Weald anticline. At its margins are the relatively flat clay vales of the Low Weald, the Greensand Ridge, the North Downs and South Downs. At the centre, the sandstones and clays formed geologically of the Hastings Beds of the early Cretaceous period rise up to give the characteristic forested ridges of the High Weald. This geology and the subsequent dissection of the sandstone core by rivers such as the Ouse, Medway and Rother are fundamental to its underlying landscape character. The headwaters of these rivers have cut deep, steep-sided valleys (ghylls) which are wet and clayey. On this underlying geological structure centuries if not millennia of human activity have created the landscape that is visible today.