Sand dunes are small hills or ridges of sand found at the top of a beach, beyond the reach of the sea. For coastal dunes to form, there needs to be a supply of sand, a means of transporting it and somewhere for it to collect more quickly than it is eroded.
Sand is created primarily from the weathering and erosion of rocks, mostly from inland and carried to the sea by rivers, but also from coastal cliffs bombarded by waves and the particles moved around by currents. Another other source is calcium carbonate from the remains of the shells of dead sea creatures. Roughly 99.7% of sand is quartz particles and inert, that is, they do not react chemically.
Sand is finer than a granule and coarser than silt, with actual definitions of its size varying but generally in the region of between 0.06 and 2.0 millimetres in diameter. Typically, a particle of sand is small enough to be carried by the wind, but large enough to fall in still air, while gravel will not be wind swept, and dust particles are seen suspended. This characteristic is important in the development of sand dunes.
At low tide, sand in the area between the high and low tide marks is exposed and may start to dry. The dry grains of sand can then be transported by the wind. A dune begins to grow when sand is blown inland by the wind and starts to collect against an obstacle such as a rock or driftwood. As more sand particles gather, the dune grows in size, forming at a right angle to the prevailing wind direction. The open bare sand provides a habitat for some of the UK’s more scarce invertebrates such as sand lizards. If not eroded, the sand may become colonised by small plants.
Salt resistant species such as marram grass and sand couch are amongst the plants to colonize such as area. They are known as pioneer plants in the growth of sand dunes. They aid the growth of the dunes by binding the loose particles underground and trapping more sand around their shoots, gradually beginning to stabilize the growing dune. More animal species are able to survive with the increase in stability and shelter.
As the dune grows, it traps more sand, but the wind helps migrate the mound by taking particles up the windward side and depositing them on the leeward side. In the Yellow (or White) Dune stage, marram grass is continually buried and regrows up through newly deposited sand. Plants such as sea holly and sea spurge can take root on the more sheltered leeward side of the dune. As the system stabilizes, the sand is enriched by decaying vegetation and more plant species can take root and the animal species diversify.
A system of dunes develops with hollows and areas of exposed sand, ensuring a continuity of open vegetation and bare ground with is free-draining and quick to warm, important for many invertebrates. Wet depressions between dune ridges are known as dune slacks, and provide an environment for still more plant and animal species. However, if the sea reaches the dunes, they are quickly eroded.
Eventually, the dune will have more or less continuous plant cover, effectively anchoring it in place. This is known as a Grey Dune, usually found further inland in a system at the back of a beach. The marram grass dies out as sand stops accumulating and a grey-green lichen forms. These ‘Calcareous’ dunes are particularly rich in species but if not actively managed can become colonized by scrub species. The decay of vegetation from previous stages means a build-up of humus in the sand which becomes capable of supporting pasture grasses and bushes. Where the dune sand lacks pulverized shall material, or have been leeched of nutrients by rainwater, the developing soil substrate will be acidic and dune heaths may become dominated by acidic loving plants such as gorse, heather and bracken. The final stage of dune stabilisation often occurs by the plantation of conifers, however, even without tree planting, left alone, the dunes would eventually develop into woodland.
Dunes are under threat due to a large number of factors - excessive pedestrian use, car-parking and use of off-road vehicles contribute to high levels of erosion as it disrupts stabilization by plants and increases movement of sand. Golf courses, such as found here, fragment the habitat and modify the vegetation. Drainage of adjacent land for agriculture and housing has an adverse effect on dune slacks which may dry out. Engineered defence systems often reduce the natural dynamics of dune systems and impact on their biodiversity. Mechanical beach cleaning removes material along the driftline, affecting both the chances for sand to accumulate and destroying the habitat of the pioneer plant and animal species. Grazing has previously kept down the invasion of coarse grasses and scrub, where abandoned, the invasive species out-compete the fixed dune vegetation. Sea buckthorn in particular can smother dunes in a dense thicket. Dune habitats have been depleted in Britain. Now, however, many dunes, including this one, are designated as Sites of Special Scientific Interest (SSSIs) and under the European Habitats Directive fixed dunes and dune heaths are regarded as priorities.
Fences and boardwalks are used to reduce disturbance from trampling by the public to the normal dune fixation processes. These have to be reviewed to prevent over-stabilization of yellow dunes. Periodic fencing to allow recovery is one way of managing this. Scrub is an important habitat and small amounts of native species should be maintained, but managed to prevent spread over wide areas. Sea buckthorn in particular needs to be actively managed to prevent covering of the areas of bare sand and drying of the site. If native woodland is present, it should be preserved. Seepages, streams and brackish creeks should also be retained. High water tables in the area need to be maintained to support dune slacks naturally.
Long-established grazing needs to be continued or established species dependent on it will perish, and the whole system be unbalanced. Rabbits can be a helpful species as long as the population does not increase to a level likely to threaten stabilization. Their grazing produces cropped lawns amongst taller vegetation, important for many species, and their scrapes provide areas of bare and disturbed ground required by others. Their dung and carrion also provide resources for many invertebrates. Sheep grazing may serve the same ends but vigilance is required to prevent over-grazing. Similarly though, in dunes where grazing has never been used, grazing may be highly damaging to the ecosystem that has involved there. No one management strategy fits all dunes.
To log the Earthcache
To log this cache, you need to visit the published co-ordinates and the additional waypoints S1 and S2. They are accessible either via the boardwalk (which is closed at times during winter if the surface becomes slippery) or around the top of the beach past the cafe building from the car-park. Have a look around as you walk. Then, send me a message through my geocaching profile with the following information:
The name and GC number of this geocache, which dates you visited and logged it, and the answers to the questions below.
1. The additional waypoints take you to the beach end of the boardwalk and a sand path further along the beach. Briefly describe the differences you can observe between surroundings of the two.
2. Identify three features in the surrounding area that might be a threat to the dunes here. Why might you consider them a threat?
3. At the published co-ordinates you will find a cylindrical object. What colours is it painted and approximately what is its diameter?
4. Optionally attach a photo of yourself and/or your GPS at the beach (but please do not show the object in question 3!)
That’s all you have to do. I hope this geocache was enjoyable.
You do not need to wait for a reply; you may log the cache as soon as you have sent the message, and I will get in touch.