Before going further, it’s important to note that my understanding of Folkestone’s geology is basic; I am first and foremost a collector, not an academic. Nevertheless, I have done my best to share what I believe to be accurate.
 

Geological Formations of Folkestone

 

Folkestone’s geology consists primarily of three formations: the Folkestone Sand Formation, 

the Gault Clay Formation and the Chalk

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The Folkestone Sand Formation

 

The Folkestone Sand Formation is a rock layer in the UK, part of the Lower Greensand Group, which formed during the Late Aptian and Early Albian stage of the Early Cretaceous period, roughly 115 million years ago. This formation is made up of medium to coarse sand and soft sandstone, notable for its high glauconite content, a mineral that gives the rock its greenish colour. Scientists think these rocks were formed in a shallow inshore environment.

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From my personal experience, the Folkestone Sand Formation is extremely hard and challenging to work with, even using tools. Fossils are not commonly found, but when they do occur, they typically include echinoids, oysters, gastropods, and ammonites. Although this formation is considered a possible source for dinosaur remains, I have only ever heard of footprints being found, not actual bones. 

 

The Gault Clay Formation

Upper & Lower

 

The Gault Clay Formation is a stiff, blue clay layer deposited in a calm, relatively deep sea around 100 million years ago, during the Early Cretaceous period. It can be seen clearly in the sea cliffs at Copt Point in Folkestone, Kent, where it overlies the older Lower Greensand.

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Most fossil finds washed out on the foreshore at Folkestone come from the Gault Clay Formation. The variety includes bivalves, ammonites, belemnites, and crinoids as the most common, with crabs and fish or shark remains being less frequent. Marine reptile fossils are rare but possible. My own experiences are solely based on collecting on the foreshore, since direct digging in the Gault Clay cliffs is prohibited due to its SSSI status.

 

The Lower Chalk

 

The grey chalk (Cenomanian) caps the Gault Clay Formation at the Copt point and continues towards Samphire Hoe to the east.  It was deposited in a moderately deep sea.  Invertebrate fossils are quite common, fish and reptile remains are quite rare.  These fossils are not included in this website but can be found following this link on Samphire Hoe

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Scientific Research and Fossil Beds

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The Gault Clay Formation has been extensively studied by academics including Jukes-Browne, Spath, Owen and Casey. Their research revealed that certain ammonites are found only in specific levels of the Gault clay, leading to the division of the clay into Beds I–XIII. This classification makes it much easier to locate specific fossils, as you’ll know exactly which layer to search.

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These beds are particularly important if you wish to donate your collection to a museum, as they provide crucial context for your specimens.

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• The mammillatum beds of the Folkestone Sand Formation are Early Albian.

• Beds I–VII of the Lower Gault Clay Formation are Middle Albian.

• Beds VIII–XIII of the Upper Gault Clay Formation are Late Albian.

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Structure and Subdivision of the Gault at Folkestone

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Below is a summary of the main beds in the Folkestone Gault Clay, proceeding from the base (oldest) upwards (youngest), with their traditional names, numbers, lithological descriptions, and typical thicknesses.

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Lower Gault

The Lower Gault is comprised of beds 1-6 and is generally more sandy and calcareous than the Upper Gault.

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Bed 1 - Basal Gault

Description: Dark bluish-grey, calcareous, somewhat sandy clay. Contains greenish glauconite grains and phosphatic nodules at the base.

Thickness: 3.13 m

 

Bed II

Description: Bluish-grey clay, less calcareous than Bed 1, with scattered phosphatic nodules and occasional shell beds.

Thickness: 1.35 m

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Bed III

Description: Firm, dark grey to blue-grey clay, fossiliferous, with bands of septarian nodules ("cement stones") and abundant ammonites.

Thickness: 1.83 m

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Bed IV

Description: Blue-grey clay, locally silty, with scattered large nodules and occasional pyrite.

Thickness: 0.15 m

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Bed V

Description: Dark grey clay, rich in fossil bivalves and ammonites. Thin beds of pyritised wood and shell fragments.

Thickness: 0.48 m

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Bed VI

(Junction Bed)

Description: Pale grey to buff, calcareous clay with a sharp, erosive base. Contains abundant crushed shells and is sometimes referred to as the “Junction Bed.” Distinctive marker between Lower and Upper Gault.

Thickness: 0.3 m

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Upper Gault

The Upper Gault consists of the thicker beds 7-14, generally less sandy, more clay-rich, and slightly darker in colour.

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Bed VII

Description: Dark blue-grey, blocky clay, fossiliferous with abundant ammonites and bivalves. Occasional ironstone nodules.

Thickness: 2.38 m

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Bed VIII

Description: Massive dark grey clay, compact, with scattered pyrite and phosphate pebbles. Fossil-rich.

Thickness: 1.33 m

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Bed IX

Description: Similar to Bed 8, with a higher concentration of fossil bivalves and thin bands of sandy clay.

Thickness: 2.85 m

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Bed X

Description: Firm, blue-grey silty clay, rich in ammonites, belemnites, and infrequent fish remains.

Thickness: 1.55 m

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Bed XI

Description: Compact, dark grey clay, fossiliferous, with rare septarian concretions.

Thickness: 10.67 m

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Bed XII

Description: Blue-grey to olive-grey clay, slightly calcareous, with thin bands of shelly material and pyrite nodules.

Thickness: 1.0 m

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Bed XIII

Description: Softer, blue-grey clay, fossiliferous with many small bivalves and scattered ammonites.

Thickness: 13.7 m

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Below is a photo of the Gault cliffs at Folkestone, taken near Copt Point on 15/06/25. I have attempted to identify the various bed levels, though due to shifting clay, many lower beds are covered by upper layers. The top three beds are not shown in this picture.

 

 

For a more detailed breakdown, I highly recommend Fred Coulter’s website, an invaluable resource for ammonite identification and general information, which has greatly assisted me in my collecting endeavors at Folkestone.

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