London Clay Formation

The London Clay Formation is a marine geological unit laid down in the Early Eocene, roughly 54 to 50 million years ago, underlying much of southeast England as a bluish clay that weathers to brown and reaches up to 150 meters thick in Essex and Kent. It is renowned for its fossils, including some 350 named plant species preserved as seeds and fruits, which point to a warm climate whose flora resembled that of modern Asia and East Africa. Its stable, easily worked clay also made it well suited to tunneling, and construction through it enabled the rapid growth of the London Underground.