The Geology around the Coastal Park

The Geology

Geo Conservation-Kent

Herne Bay Coastal Park Photography 1The short stretch of coast from Reculver to Beltinge has much to interest those who want to find out more about the physical features of the land and man‘s interaction with the landscape. The rocks beneath our feet help to explain why this stretch of coast looks as it does. This coastal section is part of the Thanet Coast SSSI (Site of Special Scientific Interest), in part because it is recognised as being of national geological importance.

This coastline has changed considerably in the last few thousand years. The former tidal channel to the east, known as the Wantsum Channel, once separated the Isle of Thanet from the rest of Kent. Sediment was actively deposited here and, following land reclamation, the land became suitable for cultivation. In contrast the cliffs west of the channel have been receding due to erosion by the sea.

About 20 metres beneath the geological beds exposed in the cliffs east of Herne Bay lies the Chalk. The Chalk was deposited on the sea-floor during the Cretaceous Period. Following the Cretaceous sea level fell, exposing the Chalk to the action of wind and water. The top-most beds of Chalk were eroded forming a gap in the sedimentary record known as an unconformity. Perhaps as much as 150 metres of Chalk was removed during this interval.

Herne Bay's Geology

The beds we see exposed in the cliffs are sands, silts and clays laid down in a marine environment about 60-55 million years ago during the Palaeocene. The shorelines at that time were constantly changing. Sand was deposited close to the land and clay settled out in deeper, quieter waters.

Marine Bi-vales at ReculverThe beds we see at the base of the cliff section near Reculver are those of the Thanet Sand Formation. They contain marine bivalves, often in shelly horizons. The sands were deposited in a low-energy coastal marine environment. There are numerous large, flat bodies of hardened sand poking out from the cliff face, known locally as ‘doggers’. Where the cliffs have been eroded by the sea, these concretions are found scattered on the foreshore.

The beds of the Thanet Formation are followed by further deposits of marine sands known as the Upnor Formation. These younger sediments represent a shallowing-upwards sequence in the cliff section, indicating a shoreline that became progressively closer. The junction between the Thanet Formation and the Upnor Formation on the north Kent coast is often indistinct and difficult to identify in the field, but is generally taken to be an unconformity (an erosion surface) overlain by a bed containing occasional nests of shells of a shallow water marine bivalve.

The sands of the Upnor Formation have been described as having a salt and pepper’ appearance. If you look carefully at the sands with a hand lens (or a good magnifying glass) you will see dark grains of the iron mineral glauconite finely disseminated through the otherwise orange-yellow (iron-stained) quartz sand.

Above the Upnor Formation lies the Harwich Formation with a basal bed comprising numerous rounded black pebbles of flint within a matrix of sand and silt. This pebble bed is a good marker horizon in the cliffs and, although neither continuous throughout East Kent nor consistent in depth, still stands out clearly in most of the visible cliff section where it is approximately 15 centimetres thick. The black colour of the pebbles is largely derived from the mineral manganese which provides a thin outer coat. The glauconitic sands above the pebble horizon contain a fossil fauna which is indicative of brackish water conditions with frequent marine incursions. The sands are often laminated (thin horizontal beds), but also show cross-stratification (sloped beds) and have been interpreted as sediments deposited by currents in a near-shore environment.

Overlying the Harwich formation are the beds of the London Clay Formation. During these times the coarser land-derived sediment continued to be deposited close to the coastal margin, which now lay far to the west, while only the very fine-grade clay material was washed out to the deeper seas, as seen in the cliffs at this location.

On a fresh exposure the clay may appear grey-blue in colour, but after contact with oxygen in the atmosphere it will turn a chocolate-brown. The clay is intensely burrowed by bottom dwelling organisms indicating the sea-floor was well oxygenated and a hospitable environment for marine life. Some of the fossils in the London Clay Formation are pyritised and small grains of iron pyrites can also be found finely disseminated throughout the clay. Iron pyrites (FeS2) is commonly known as fool‘s gold.

Over the next 55 million years or so all the sediments were buried and compacted and underwent various chemical changes The cumulative affect of millions of years of burial was to alter the coastal sands and muds into the rocks we see preserved today in the cliffs and on the foreshore.

During the Ice Age, Kent was not glaciated, but the ground surface was frozen for much of the year and was a similar environment to a modern tundra. Annual cycles of freeze and thaw took place, which could churn up the surface sediments to a depth of several metres. Periodically small, north-flowing streams rising in the higher ground to the south occupied shallow valleys across the landscape. Many of these streams flowed during interglacial periods. The streams deposited sands and gravels in their channels some of which can be seen from the foreshore exposed near the top of the cliff section. Within the gravel deposits between Herne Bay and Reculver stone tools dating back to the Palaeolithic (12,000 to 2,600,000 years ago) have been found; evidence that our remote ancestors once walked this landscape during an interglacial period.

Also resting uncomfortably on top of the cliff sediments, and sometimes lying within depressions of the former land surface, can be seen a pale brown-coloured horizon known as 'brickearth'. At Bishopstone Glen three to four metres of this bed can be seen below the soil horizon and immediately overlying the much older sands and silts. Most brickearths are wind-blown deposits known as loess, dating from the Ice Age. Brickearth has been quarried extensively in Kent, notably around Faversham where it has been used to make stock bricks for many years.

Important note: the coastline is actively eroding and the cliffs are unstable and dangerous. Cliff falls are common. Do not approach the cliff face and, when walking along the cliff top, do not approach the edge.

We are indebted to GeoConservation Kent for this article and photographs.

GeoConservation Kent cares about geological sites and seeks to identify, conserve, enhance and research those of particular importance in Kent and have produced two booklets describing the geology of the coast between Herne Bay and Reculver.

The Geology of Reculver Country ParkThe Geology of the Reculver Country Park (54 pages) introduces the reader to the geology of this section of the north Kent coast and explains how the local landscape has evolved over the past few thousand years.

 

 

The Stones of Reculver Country ParkA companion booklet The Stones of Reculver Country Park (42 Pages) describes the two ancient scheduled monuments at Reculver and the different building materials used in their construction. This booklet also describes the rocks that have been introduced to protect this section of coastline from further marine erosion.

 

Both booklets contain a guided walk and are illustrated in colour throughout. Proceeds of sale go towards geological conservation in Kent.

Each booklet is £4.50 inclusive of postage and packing or both booklets can be purchased at the discounted price of £8.00 inclusive of postage and packing. Cheques to be made payable to “Kent RIGS”.

Copies are available from GeoConservation Kent, 6 Manor Close, Canterbury, Kent CT1 3XA.

Please visit our website at www.geoconservationkent.uk and visit our Facebook page

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