Proper noun
Oxford Clay
A Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire.
Oxford clay makes up the rest of the borough. citation The River Ray rises at Wroughton and forms much of the borough's western boundary, joining the Thames which defines the northern boundary. Source: Internet
The Vale of Pewsey has been cut through the chalk into Greensand and Oxford Clay in the centre of the county. Source: Internet