Geological Field Visit to Osea Island, Maldon, Essex
[TL 915058]

Sunday 28th June 2009

Introduction & Aims: Twelve friends and members of the Essex Rock & Mineral Society, Tertiary Research Group and Essex Field Club mustered at Decoy Point at about 9.15am. The majority had earlier parked in the car park of the Mill Beach Public House and walked along the sea wall footpath. The day started rather overcast and cool but during the morning the sun started to shine and it became rather hot and humid. The visit had three main aims. Firstly to inspect the cliff and foreshore exposures of the London Clay Formation (lower Eocene age c. 53 million years) on the south side of the Island between the pier and West Point. Secondly to look at the Pleistocene gravels and brick earth intermittently exposed at the top of the beach and in the low cliff. The third aim was to search the foreshore at West Point [TL 904066] for late Neolithic–Early Bronze Age 2300-140BC flints. Graham Ward kindly produced and distributed a hand out. This was not the first trip to Osea by the Essex Field Club. They made a fascinating visit here on 15th September 1888 when they paid particular attention to the flora, shells, seaweeds and worms. On that occasion Mr T.V. Holmes gave an account of the geology of Maldon and the Blackwater Estuary (Cole 1888).

Members of the Essex Field Club exploring Osea in 1888.

Drawn by H.A. Cole

Background: Local myth states that the long causeway, about a mile in length, across the mudflats from Decoy Point to the Island was constructed by direction of a Roman Governor. It is very well maintained and passable for about 4 hours at low tide, but this time varies considerably and extreme caution must be exercised in crossing bercause the causeway is submerged to a depth of about 3-4 metres at high tide. The mudflats can be rapidly flooded as the tide flows.

Crossing the Stumble from Decoy Point to West Point, Osea

Osea Island is private property and visitors must keep below high water mark at all times. The word ending “ey” or “ea” used to mean island as in Chelsea or Battersea or ayot for an island in the Thames. Pedantically Osea means Osyth’s Island so Osea Island is technically tautology. It appears that both Northey and Osea Islands were given in about 1180 to Beeleigh Abbey by Robert de Mantell. In medieval times the island had a decoy pond to catch wildfowl. In 1903 the Island was bought by the repentant brewer Frederick N. Charrington as a temperance centre, but, the inmates smuggled in alcohol, and locals rumour records that caches of buried empty beer bottles are often being dug up. During the Great War of 1914-1918 Osea was used by the Royal Naval Volunteer Reserve. The navy even built a chapel here for them. Fast Coastal Motor Boats, with a speed up to 55 knots, were manufactured here and first used in 1918. After the war the Island returned to arable farming. The Island was bought by Hilary and David Cole in 1960 for £70,000. They sold it to Cambridge University in 1968 but bought it back in 1986 and started to plant 3,000 trees. The most imposing building is the Edwardian “Charrington House” or Rivermere constructed in 1910 as a convalescent home for drunkards and inebriates. It has now been converted into ten luxury apartments normally occupied by journalists and literary agents. The island was recently up for sale for £6 million (Fautley & Garon 2005 pp. 100-103).

Searching for London Clay Fossils on the foreshore west of the Pier, Osea

Geology: The basic geology of the Island is quite simple. H.B. Woodward geologically mapped Osea and recorded that it "has a capping of gravel 4 to 5 feet thick, which forms a low cliff, resting on London Clay”. He drew the north-south section below (Whitaker 1889 p. 453).

A well was sunk by J.J. Furlong, at Mr. Charrington’s Manor House, then being used as an Inebriate Asylum, in 1904 to a depth of 176 feet as follows:

Water was found at 156 feet and rose to 21 feet down. The water was analysed by Dr. Thresh in 1904. The yellow clay underlying the gravel, interpreted by Whitaker as being London Clay, is probably a mixture of brick earth and weathered London Clay.

London Clay: The London Clay exposed on the foreshore and lower part of the cliff belongs to the base of palaeontological subdivision Unit 8 within Lithologic Unit C as determined by the British Geological Survey (Bristow 1985 p. 23). This is indicated by the fossils Isselicrinus subbasaltiformis (Miller), Hoploparia gammaroides M’Coy; Linuparus scyllariformis, Teredina sp., gastropods, scaphopods, and fish teeth (George and Vincent 1976 p.27; Bristow 1985 p. 23.).

Members of the party searched the foreshore and soon found numerous Isselicrinus crinoid stems and some lobster fragments of Hoploparia and Linuparus.

Hoploparia	Isselicrinus
Source: British Caenozoic Fossils 5th edition 1988 Plate 4

The London Clay may is currently poorly exposed on the foreshore. It rises up to 2m in the cliff in places and has been cryoturbated or disturbed by ice pressure. Shattered cement stones may be seen at the base of the cliff and on the foreshore.

In situ London Clay Cement Stone, Osea, Essex

Gravel exposed either side of London Clay in low cliff at Osea, Essex

Pleistocene: Gravel and brickearth may be seen overlying the London Clay at the top of the beach and in the low cliff. In the Blackwater Valley five river terraces have been recognised and mapped by the Geological Survey. At the time of the geological mapping, 25 years ago, the age of these different terraces was uncertain. According to Bristow (1985 p. 65; 75) on Osea the 2nd Terrace rests on deposits of possible Hoxnian age. At the western end of the Island the 2nd Terrace gravels rest on grey silty clays of uncertain age (Bristow 1985 p. 74; p.79). They have been mapped as 2nd Terrace Loam, but, they may be Hoxnian Interglacial or Glacial Lake Deposits. Apparently the surface level of the terrace varies from 4 to 7m OD. Its base on the foreshore is only about 2 to 3m above OD. The Geological Survey also mapped a tract of 1st Terrace (Loam) [910 062]. This consisted of up to 1.8m of grey, brown and yellow silts and silty clays overlying deposits mapped as 2nd Terrace Loams (Bristow 1985 p. 80). Several rather worn Palaeolithic flakes were found in the beach gravel. A fine specimen was collected by John Everett.

Palaeolithic flake collected by John Everett from beach gravel on Osea

Recent: Storm Gravel Beach Deposits (Cheniers) are present almost continuously around the island and are detailed by Bristow (1985 pp. 84-85). Members of the party searched the beach at West Point (TL 904066) for Late Neolithic – Early Bronze Age (2300-1400BC) flints (Vincent & George 1984). Several flakes and a scraper were found.

Early Bronze Age tanged and barbed arrow heads from Osea Island, Essex

(Vincent & George 1984 figs. 2-4)

Side scrapers from Osea Island (Vincent & George 1984 figs. 27 & 31)

In Neolithic times Osea would not have been an island, but would have been situated to the north of the Blackwater Estuary. In the late Mesolithic period the head of the Blackwater Estuary was probably just to the south east of Osea (Wilkinson & Murphy 1995 pp. 211-213).

Storm Gravel Beach Deposit, Osea Island

Conclusion: Members of the party managed to examine some exposures of London Clay in the low cliff and on the foreshore and collected some fossil crinoid stems and lobsters. The Pleistocene Gravels and brick earth in the cliff were inspected and a few rather worn Palaeolithic flakes were found loose in the beach gravel. Finally a few Bronze Age/Neolithic flakes were found at West Point. Accordingly the aims of the field trip were successfully accomplished. The majority of the party returned to Mill Beach as the tide swiftly flowed back over the causeway. The directors retreated to Maldon to slake their thirsts in a local hostelry before adjourning to the Wilkins Jam factory at Tiptree to partake of a delicious cream tea.

Heading back to Decoy Point from Osea, Essex

References:

Bristow, C.R. et al. 1985.Geology of the country around Chelmsford. Memoir for 1:50,000 geological sheet 241. ISBN 0 11 884335 4. 108 pages.

Cole, W. 1888. Visit to Maldon and the Blackwater Estuary, September 14th and 15th 1888. Essex Naturalist Vol. 2 pp. 228-255. [Osea pp. 249-251].

Fautley, M. & Garon, J. 2005. The Essex Coastline – then and now. Second Edition. ISBN 0 9548010 0 8. 272 pages.

George, W & Vincent, S. 1976. Some river exposures of London Clay in Suffolk and Essex. Tertiary Research Vol. 1 pp. 25-28.

Vincent, S.W. & George, W.H. A Late Neolithic-Early Bronze Age: 2300-1400BC flint assemblage from Osea Island, Essex. 1984. 11 pages.

Whitaker, W. 1889. The Geology of London and part of the Thames Valley. Memoirs of the Geological Survey. Vol. 1 Descriptive Geology. 556 pages.

Whitaker, W & Thresh, J.C. 1916. The Water Supply of Essex from Underground Sources. Memoirs of the Geological Survey. 510 pages.

Wilkinson, T.J. & Murphy, P.L. 1995. The Archaeology of the Essex Coast, Volume 1:The Hullbridge Survey. East Anglian Archaeology No.71. ISBN 1 85281 119 7. 238 pages.

W.H. George. 21.10.2009

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