Oxford Clay Formation

The Oxford Clay, or Oxford Clay Formation, is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset to as far north as Yorkshire. It dates to the Callovian and Oxfordian ages, roughly 165 to 159 million years ago, and is composed mainly of claystone and mudstone, divided into a lower, organic-rich Peterborough Member and an upper set of more calcareous, less fossiliferous members, the Stewartby and Weymouth Members. It reaches up to 185 meters thick at its maximum, though it is typically 50 to 70 meters thick on the East Midlands Shelf.