Geology
Here is the British Geological Survey map in grainy close-up. The brown area marks Weald Clay and the yellow shows the extent of Upper Tunbridge Wells Sand. The northern part of Lingfield Wildlife Area lies under the wording "Lingfield Common".
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Running through the region, marked by the cream colour, is a strip of Recent Alluvium (recent in geological time, that is). Alluvium, from the Latin alluvius, meaning "washed against", is material deposited by the passage of a stream or river.
In this case, as you can see from the map, a strip of alluvium follows the routes of the River Eden and its tributaries. These include Gibbs Brook, Eden Vale Stream, Eden Brook and the Felbridge Water. Not all of them appear here.
The River Eden rises south of Caterham and eventually runs into the Medway near Penshurst.
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The Lower Cretaceous
All these substrates are part of the Wealden Series of rocks and clays. These were laid down about 120 million years ago during the Lower Cretaceous period, which lasted from about 140 to 65 million years ago.
The British Isles were then about 40 degrees north of the equator, more or less where Spain is today. They were a tiny part of the supercontinent of Laurasia. This huge land mass comprised North America, Greenland and most of Eurasia. It did not fully separate into its component parts* until some 25 million years ago.
Before then, Laurasia had been part of the even larger land mass, Pangea. This split 200 or so million years ago, forming Laurasia and Gondwanaland. The latter comprised Africa, South America, India and Antarctica.
(*Some geologists say that events were the other way around, and that Laurasia collided with Gondwanaland, forming Pangea. It's so helpful to the rest of us when experts can't even agree over something as fundamental as that!)
There is a dramatic animation of the sequence of events in Britain at this BBC Web site. (You'll need to have Macromedia Flash on your browser first.) For a more detailed discussion of Earth's history, the University of California at Berkely, USA, has a superb site here.
Dinosaurs
The Wealden Series was deposited while brackish lakes covered south-eastern Britain. Forested mud swamps were home to several large dinosaurs. These included the herbivorous Iguanodon, whose bones were first discovered about 180 years ago at Cuckfield, West Sussex, 15 miles away from Lingfield. There's more about it at the BBC's Walking with Dinosaurs pages and at the Zoom Dinosaurs site for children (scroll down past the first part-page).
Another local dinosaur was the related Valdosaurus canaliculatus - literally the groovy Wealden lizard! (The groove is in the end of its thigh bone.) This was discovered only in 1975. There's more about Valdosaurus here and a list of dinosaurs found in the Wealden Beds here.
Other fossilized remains from the Weald show that the insect life of the time would be recognizable today. It included dragonflies, damselflies, caddis and stone flies, beetles, wasps and cockroaches.
All these creatures, large and small, fed off each other and off the abundant plant life. This was changing. The primitive giant mosses, ferns and horsetails of the Carboniferous Era, whose compressed remains we burn as coal, were evolving into more complex, flowering plants. They were able to use the insects to help with pollination. Magnolia, which was native to Britain million of years ago, was one of the first species to do this.
What lies under the pond
In late 1997, before we dug our pond, Tandridge District Council took a core sample. It also sank a small exploratory shaft in that part of the fields. Partly this was to learn more about the underlying geology and partly to see whether the hole would hold water (it did). The report said this.
The total depth of core was 1.75 metres.
Top soil encountered was less than 150 mm deep.
The first 900 mm of core extracted was of an orange and grey mottled clay of varying proportion, with some haematite and occasional silty bands. Below this it became a stiff orange and grey mottled silty-clay with sand lenses, quartz and iron nodules. It should be noted that the trial hole had a hard continuous ironstone band at 1.3 metres, although evidence of this was not found in the core site 1O metres away.
Due to the nature of the sediments it is expected that the top 900 mm (less the topsoil) would be suitable to form a pond. The material found below this is inconsistent in lithology and is likely to be difficult to excavate should the ironstone be encountered.
The "orange and grey mottled clay" is typical of the area; geologists give it the macabre nickname of 'catsbrain'. It is associated with Weald Clay, which looks to have spread further south than the map above suggests. Weald Clay was much used for bricks and there was a brickworks about two miles away, at Crowhurst. You can see the works' remaining chimney from the reserve.
Haematite and ironstone are, of course, iron-bearing rocks. The Weald was famous as an iron-making area from before Roman times until the late 18th century. Fortunately for us, the ironstone in that corner of the fields was too scattered to obstruct excavation.
St Margaret's Gate
There is further evidence of iron in the silted-up pond just north-east of St Margaret's Gate, at the top end of the reserve. (The gate appears on the plan, here.) It is fed by a chalybeate, or iron-bearing, spring. This causes the typical brown colour of the water and the iridescent film on its surface.
The word 'chalybeate' - pronounced ka-libby-et - derives from the Chalybes, a people of Pontus, in Asia Minor (Asiatic Turkey), near the Black Sea. They were famous from the 15th century BCE for working in iron and steel. The soil and sands in that area are full of iron salts.