 The ridge is pierced by the rivers Wey and Mole, tributaries of the Thames, which formed the northern border of the county before modern redrawing of county boundaries, which has left part of its north bank within the county.To the north of the Downs the land is mostly flat, forming part of the basin of the Thames.The geology of this area is dominated by London Clay in the east, Bagshot Sands in the west and alluvial deposits along the rivers.