Geology of Surrey

During the Early Cretaceous epoch (from about 145 to about 100 million years ago) Surrey alternated between a fresh-to-brackish water embayment depositing Hastings Beds and Weald Clay, comprising shales and mudstones that are often finely banded.

Offshore muds (now shales and mudstones) of the Atherfield Clay were deposited followed by shallow marine sands of the Hythe, Sandgate and Folkestone Beds.

Where not eroded to lower heights, there is then a marine layer of the sands of the Hythe Beds topped by chert seen on today's remaining Greensand Hills.

The Gault Clay contains phosphate-rich nodules in discrete bands and has a rich marine fauna with abundant ammonites, bivalves and gastropods.

[1] Just before the Paleogene, which included the mass-extinction event of the non-avian dinosaurs, sea levels dropped, exposing Sussex and Kent, and the marine Upnor Beds were deposited in Surrey.

In the Paleogene, Southern England rose slightly, and the seas retreated, and the reddish and mottled clays of the Reading Beds were deposited by a large river sand delta system.

Continuing with the Downs, in a broad band, the far northern type soil slopes such as near Croydon come across the Surrey border at Banstead and Ewell, which is free draining, slightly acid but base-rich soils, producing extremely fertile pastures and deciduous woodland, which becomes the edging for both sides of the high escarpment at Box Hill and is just found for a brief while in a middle section in Polesden Lacey then throughout the northern slope carrying on westward.

Chipstead, Banstead and Tadworth have the first free draining slightly acid loamy soil that tops the wider downs to Guildford and is found around Dorking and the across the Vale of Holmesdale north of the Greensand Hills.