 In recounting the building practices of the local area, Astbury notes that major structures such as the monasteries at Ely, Thorney and Crowland were built on islands or ridges.These structures rest on Cretaceous Lower Greensand that overlie Jurassic period Kimmeridge on top of Jurassic period Ampthill clays.For example, the village of Little Thetford, south of Ely, lies at about above sea-level, and sits largely on an island of Kimmeridge Clay, a Jurassic shallow-water shelf-sea deposit.