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South Lodge Pit

Were you aware that on your doorsteps sits a Site of Special Scientific Interest (SSSI)? Do you have any idea what lies behind the grey fencing on Mill Lane, close to the Jubilee River? No? Well, you’re not alone. The site in question is an old abandoned chalk quarry called South Lodge Pit. It is one of only 11 SSSIs in Buckinghamshire that are listed for their geological importance.

The South Lodge Pit was first noted as an important site in 1951 and granted SSSI status in 1975. The site is owned and managed by the Taplow Court estate, but the responsibility to protect the site lies with Natural England. In 1999 a significant clean-up occurred and the quarry faces were cleaned and undergrowth removed. However over the last nine years the vegetation has grown back to the point where many of the features are no longer accessible or visible.

On 31 May, the Bucks Earth Heritage Group, a small but enthusiastic group with an interest in conserving and promoting geology within Buckinghamshire, organised a quarry clean-up. Together with local volunteers and staff from Taplow Court they spent the day removing shrub and buddleia growth from the quarry faces and improving access.

Inspecting the chalk

South Lodge Pit first appears in the scientific literature in 1891 when Mr A Strahan was examining some rock samples from the quarry that had been sent to the Museum of Practical Geology in London. He recognised the unusual character of the chalk and he subsequently visited the pit.

In records from the time, South Lodge Pit was already referred to as an ‘old quarry’, possibly being abandoned for some time. The chalk was most likely extracted for the manufacture of lime for building and for agriculture. Quarries like South Lodge Pit, close to the Thames, may well have been the source for lime used in the early construction of London. There is documentary evidence of mineral extraction and quarrying since the 16th century. In the Hampton Court account books for the 1530s there is a record of burnt lime and bricks being transported down the Thames from a Taplow quarry.

To understand the significance of the pit we need to understand how the chalk formed. During the Late Cretaceous period (The Age of the Dinosaurs, 100-65 million years ago), sea level was up to 250m higher than present day. There were no ice caps and the planet was much warmer. The oceans spilt over on to the continents and most of Britain including, most importantly for our story, Buckinghamshire, was submerged below the sea. The Taplow chalk formed in these shallow marine conditions through the accumulation of the microscopic skeletal remains of millions of marine plankton called cocolithosphores. The process was very slow and it is estimated that chalk formed at an average rate of about 1 to 3 cm every 1,000 years. The low percentage of other minerals in the Taplow chalk is explained by the vastness of this Cretaceous Ocean and the distance the Taplow area was from any land at the time.

The occurrence of phosphatic deposits within chalks of this age is well recognised in north-eastern France, but South Lodge Pit remains the only site in southern England where these deposits are well developed and their age can be clearly determined. The deposits contain abundant brown granular phosphate, largely consisting of phosphate-filled and coated foraminiferal tests (the shells of another microscopic marine animal), phosphatized macrofossil fragments, faecal pellets, phosphatised intraclasts and vertebrate remains.

In 1905 Mr Harold White and Mr Llewellyn Treacher published an extensive paper describing the Taplow pit, the phosphatic deposits and the fossils found there. They recognised the beds of chalk were dipping more steeply and in a different direction to rocks from the surrounding area. Although their interpretation of the presence of folded rock strata was wrong, their observation of increased bed dips was correct. It is now thought that they represent the sides of a submarine channel feature called a ‘cuvette’.

So where does the phosphate come from? Phosphorus is abundant in living organisms; for example a average adult male will have approximately 700 grams of phosphorus in their body and will intake and excrete approximately 1 to 3 grams per day. In the marine environment, phosphate is supplied predominantly by the breakdown of marine organic matter, plants and animals. The accumulation of a phosphate deposit requires the maintenance of specific conditions for extended periods of time. The cuvettes or channels provided this environment. The surrounding marine conditions encouraged marine life, in particular burrowing creatures which brought sediments into their burrows, resulting in preservation of material that would otherwise be quickly destroyed on the sea floor. The cuvettes, or channels, became important as a means of concentrating the phosphatic materials as the finer chalk particles were transported away by the channel currents.

Were the phosphates ever extracted? As far as records indicate, the Phosphoric Beds at Taplow were never exploited commercially, even though, during both World Wars the government analysed the deposits in the quarry and found 5-20% Phosphate. The interest was probably related to the use of Phosphorus in the manufacture of munitions. However, they didn’t mine it – possibly because it’s limited to an area of about 300 by 1500m and presumably uneconomical.

More work on South Lodge Pit will be required and the group will be planning another clean-up activity in 2009 and a geological visit later in 2008 subject, of course, to permission from Natural England and the landowner. If you are interested in learning more about the local geology, why not come and join us?

Graham Hickman and Roisin Lakings