 Local clay has been used for brick-making of Fletton style bricks in the Marston Vale.Glacial erosion of chalk has left the hard flint nodules deposited as gravel—this has been commercially extracted in the past at pits which are now lakes, at Priory Country Park, Wyboston and Felmersham.The Greensand Ridge is an escarpment across the county from near Leighton Buzzard to near Gamlingay in Cambridgeshire.