The Natural History of Devil's Dyke

Devil's Dyke is a V-shaped dry valley on the South Downs in Southern England. The dyke gets its name from a creation myth, which spins a tale about how the Devil hatched a dastardly plan to carve out a valley in order to flood Sussex with sea water. To most people, this story is irrational supernatural codswallop. The land gradient from the coast to the South Downs serves to impede sea water from flooding Sussex, a reality which buries any allegation that the formation is the consequence of a non-naturalistic agent. Local folklore contrives to explain away this inconvenient fact with some close reasoning: an old woman with a candle disturbed the deity and woke a rooster, whose call, in turn fooled him into the belief that dawn was fast approaching. Afraid that others would know his presence, the Devil fled, leaving his "work" incomplete.

Devils Dyke North

It is a misconception that the valley was formed by glacial action. The Devil's Dyke V-shaped dry valley is the result of solifluction and river erosion. More than 14000 years ago, the area experienced an intensely cold climate (but not glacial conditions). Snowfields capped the South Downs and permafrost conditions meant that the chalk bedrock was permanently frozen. In summer, the snowfields partially melted and saturated the top layer of soil, because the melt water could not permeate the frozen chalk underneath. Waterlogged material situated above the permafrost slid down the gradient, removing material by friction, exposing deeper layers of frozen chalk. When the Ice Age ended, the snowfields covering the South Downs melted completely, and rivers formed across Sussex. The Devil's Dyke valley was completed by one such river.

The geological history of the South Downs extends back to the Cretaceous period, which ended about 65 million years ago. The chalk of the South Downs is fossil rock, rich in the remains of calcareous sea-creatures, the deaths of countless non-extant animals holding up the land from Eastbourne to Midhurst. The South Downs is renowned for its Adonis and Chalkhill blue butterflies. The South Downs is home to Roe deer, and isolated herds of Fallow deer too.

Early Purple Orchid 2

An Early Purple Orchid (Orchis mascula) captured in May. I achieved a smooth bokeh by setting my Canon 100 - 400 mm IS USM L telephoto zoom to 400 mm, using the widest available aperture of f 5.6.

devil's dyke valley

View of Devil's Dyke, taken from a field just 300 metres from the dry valley. Measuring 100 metres deep, the sight of this valley is a breathtaking example of how the natural world can shape and transform the landscape. During the Cretaceous period, the whole area was 200 metres under the sea. Algae living in the sea died, depositing calcium carbonate (chalk) on the sea bed
. Around 50 million years ago, during a phase called the Alpine orogeny, the African and Eurasian tectonic plates collided, which led to the formation of the Alps. Around 20 million years before present, pressure exerted on sediments caused the tectonic uplift of chalk from the sea bed, leaving the Wealden chalk dome exposed. The phrase orogeny is Greek for "mountain generating". Subsequent erosion, especially by solifluction, and summer flooding under tundra conditions exposed what is now the Sussex Weald by removing chalk. The South and North Downs and the cliffs extending from Brighton to Eastbourne are the only remains of the chalk dome.

Grass Snake Shed Skin

One of my favourite locations near Devil's Dyke is a medium-sized field, left fallow (very fallow), situated parallel to Saddlescombe road, and about 300 metres north-east from the bottom of the valley. To the south-west is a series of shallow tributaries, which support dragonfly larvae, and water boatmen; a short walk up an embankment, and over a wire fence leads to this flora and fauna rich field, complete with grass snakes (Natrix natrix helvetica) (see above image for a shed skin).

Common Blue

I took this photograph of a Common Blue (Polyommatus icarus) male just as the sun dipped under the horizon on a balmy July evening. The conjunction of a dark background, and the warm glow of the sun accurately represent the prevailing conditions in situ. The Common Blue is perhaps the most abundant and widely-distributed of the family Lycaenidae. 75% of the lycaenidae family, including the Common Blue, have a myrmecophilous relationship with ants. In a show of reciprocal altruism, ants treat the butterfly larvae as their own, allowing the animal to live in their nests, while in return, the larvae secrete honeydew, which ants use as a source of food. Ants will also attend the olive-brown chrysalis, and transfer it to their nests.

Common Blue pair

Two Common Blues about to mate during a mid-July evening. There are two broods of this species each year: the first is on the wing between May and June, and the second from July to September. The offspring from this pair probably emerged the following May, on the provision that their eggs and subsequent larvae survived predation, and other environmental factors.

Six Spot Burnet Moth

I discovered this pair of Six-spot Burnet moths (Zygaena filipendulae) mating on a blade of grass.

rabbit close 2

This European rabbit (Oryctolagus cuniculus) poked his or her head from a burrow at the foot of Devil's Dyke. I spent two hours lying prone waiting for a decent photo opportunity. The sound of my image stabilizer meant that I received two seconds in order to accomplish it.

Woodland Spring

The permeable chalk of the South Downs permits the development of aquifers, and subsequently the formation of elegant springs, like this one in a woodland just 300 metres from the foot of the valley. This particular spring floods in times of prolonged heavy rain, although the local soil may saturate anyway following a storm. Aquifers, like the South Downs need time to accumulate water reserves, and so the build-up of water may not be apparent at springs, until some days or weeks after the initial wet weather. The local water company extracts water from a number of locations at the foot of hills in Sussex; boreholes are drilled into the chalk, and the groundwater pressure forces water through the chalk for collection, and subsequent purification at nearby plants. In recent decades, inconsistent rainfall, and short deluges of heavy rain meant that the local water company struggled to meet the demands of local residents: for the optimal human exploitation of aquifers, these dynamic systems must be fed with consistent spells of precipitation, which soak into the chalk, rather than torrential downfalls that simply flow down the hills, flooding land, and tarmac.

Although chalk is efficient as a porous rock, it is inefficient as a source of extractable water. The fine-grained matrix of calcareous shells and shell fragments of plankton results in high porosity (45%) but also high levels of immobility, since under normal conditions, capillary forces render about 99% of water held within the rock as non-extractable. As a consequence, the amount of water yielded by chalk when drained naturally, or by pumping is only 1% of the total water stored within the chalk aquifer. Even so, the amount of water yielded by Sussex pumping stations is usually sufficient for local demands, because most of the extractable water held by the South Downs comes from a network of permeable cracks within the chalk.

All images and text are © Alan Mackenzie with the following exception. Since the text in paragraph two of this article appears in the Wikipedia entry for Devil's Dyke, paragraph two is released to the public domain under the GNU Free Documentation License.

Technical details: Canon EOS 3 with a booster pack, Canon macro speedlite, Fuji Veliva, Canon 100mm Macro, Canon 100 - 400mm L USM Image Stabilizer, Sigma 28 - 70mm f2.8 EX, Manfrotto tripod. Location: Devil's Dyke, United Kingdom.

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2 Comments - Post a comment:

Kay said...

Thank you for your article here. I am hosting a visit by a prominent aboriginal artist over to accompany an exhibition which is coming to London. I want to bring her to Devil's Dyke, as the nearest natural site to Brighton. As a result I am doing a bit of research as I am not really familiar with the mythology or the geology of this place and your writing is being very helpful. Do you know of any sacred or special places that you would recommend me showing my guest? It needs to be accessible as she is an elder and although she probably has more stamina than me I don't want to over extend her.

Kay

Alan Mackenzie said...

Kay,

I am glad that you find my site useful. I not only love taking natural history photographs, but I strive to understand the science behind my subjects.

As a result I am doing a bit of research as I am not really familiar with the mythology or the geology of this place and your writing is being very helpful.

If there is anything else you want to ask me about Devil's Dyke, please let me know.

Do you know of any sacred or special places that you would recommend me showing my guest?

I would recommend the Chatri, a Great War memorial donated by Indians to Brighton, as a thank you using The Royal Pavilion as a hospital for their wounded servicemen. You will find the Chatri about 1 mile north of Patcham. Just cross the bypass, near Asda, and walk north along the Downs, if she can manage it.

Regards,

Alan.