Geology Site Account

Site name: Sandmartin Cliff

Grid reference: TQ 6075 7919

Brief description of site:

Fine vertical cliff of Thanet Sand with the junction with the Chalk at the base of the cliff (usually obscured by fallen sand). Site of geological interest with potential for promoting geology. Part of Chafford Gorges Nature Park.

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Site details

Just north of Grays Chalk Quarry SSSI (on the other side of the road), a fine cliff of Thanet Sand can be seen on the corner of Drake Road and Devonshire Road next to the Sandmartin public house. At the base of the Sandmartin Cliff is the junction with the underlying Upper Chalk, the junction being marked by the characteristic Bullhead Bed (see below). The flint nodules from the Bullhead Bed still have their spiky protrusions (bullheads) and they are coated with the dark green mineral glauconite making them instantly recognisable. This junction is called an unconformity and it represents a missing time interval of several million years. This interval includes the Cretaceous/Tertiary boundary (also called the K/T boundary), a time of mass extinction around the world which witnessed the end of the dinosaurs and over 60% of all species on Earth at the time. At the time of writing this junction is unfortunately obscured by sand that has accumulated at the base of the cliff and the growth of vegetation.

At the top of the cliff there is Orsett Heath Gravel laid down as the highest and oldest terrace of the modern Thames. The height of the gravel, around 30 metres (100 feet) higher than the present river, proves that the Thames originally flowed at this altitude some 380,000 years ago. The land in front of the cliff is a nature reserve owned by Essex Wildlife Trust and forms part of Chafford Gorges Nature Park. The gate to the reserve is normally locked; a the key is available from the Chafford Gorges Nature Park Visitor Centre.

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The Bullhead Bed

At the junction of the Thanet Sand and the Chalk is a layer of unworn flint nodules and rounded flint pebbles in a sandy clay. The junction is often undulating and in some places the layer continues down to line the sides of solution pipes in the Chalk. This layer is the residue left behind after a thickness of chalk has completely dissolved and this may have happened after the Thanet Sand was deposited. The flint nodules from the Bullhead Bed still have their spiky protrusions (bullheads) and are coated with the dark green mineral glauconite (a potassium and iron silicate) which is formed in shallow seas. These odd coloured flint nodules are certainly unusual. In the Sudbury district, where the Bullhead Bed is also exposed, the nineteenth century quarrymen referred to them, rather unflatteringly, as Devils dung.

Often the flints are fractured and broken and some have neatly-chipped edges which show a remarkable resemblance to artificially worked stone tools. However, humans cannot have worked these flints as this layer is about 50 million years old. The fractures must be entirely due to the fact that the stones are touching and as the chalk below was slowly dissolved they were crushed together as the bed subsided. Kenneth Oakley in his book Man the Tool-maker (pages 7 and 12) illustrates two flints from the Bullhead Bed of Grays and uses them as examples of naturally flaked stones that, if seen out of their geological context, could easily be mistaken for human workmanship. The argument about the difference between natural and man-made flaking of flint was at the heart of the Eolith controversy in the early twentieth century and Essex amateur geologist Samuel Hazzeldine Warren used Bullhead Bed flints from Thurrock as a powerful argument against eoliths being produced by humans.

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The Chafford Hundred Sandmartin cliff in 1996. The important junction with the Chalk is visible here at the base of the cliff but it has subsequently been obscured by the growth of vegetation. Photo © G.Lucy