sentences:  These continue in a westerly and south-westerly direction to become the Rayleigh Hills, in Rayleigh and Thundersley.The basin is approximately in diameter, and its base consists of London Clay covered by a layer of sands, gravels, and river brick-earths, dating from the Upper Pleistocene period.The hills also consist of London Clay in their lower levels, but are covered by Claygate Beds, between thick, and where they are highest, by Bagshot Beds.