To the north of the Downs lies a syncline or basin of deposition - in geological terms this is commonly referred to as the London Basin. Although London Clay fossils tend to be infrequent finds within the fossil collections of Surrey's museums they are in fact common almost anywhere the clay is exposed in excavations or quarries. Cimonia imperialis is sometimes found along with bivalves, gastropods, crustacea, fossil wood, fish teeth, and on occasions mammal bone. In the NW of the county Middle and Upper Eocene clays and sandstones form the heathland of Surrey Heath and Chobham Ridge and also outcrop to the east at Runnymede, Chertsey, near Weybridge, and as far east as Painshill Park. Here they probably don't exceed 100 ft. in thickness.
Fossils are rare and are restricted to plant fragments and marine shells.
An ironstone band at the base of these beds was once worked as a source of iron at St. Georges Hill near Weybridge, whilst red clays were quarried for brickmaking near Bagshot. These produced the so-called Bagshot or Rubber Bricks. Fossils such as bivalves, gastropods, and nummulites are occasionally found. Georges Hill and Chobham Ridges where these are best developed.