 Above the Lias is the Lower Oolite Series which are chiefly clays and oolitic limestone . The famous Bath Stone is obtained from the Great Oolite bed . Oxford Clay is the chief member of the Middle Oolite Series ; and above this are the Upper Cretaceous rocks with Gault , Upper Greensand and Chalk . Alluvial flats and peat bogs occupy much of the centre of Somerset . 
