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Natural Environments: Stone Walls
Bradford on Avon Hundred area, Wiltshire
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Fields of the upland parts of the Bradford Hundred were traditionally separated by dry-stone walls -that is, walls that are made without the use of any mortar. In settled areas, like Bradford, the walls are more likely to be held together with mortar.
The stone was normally available on site, or very nearby, because the cost and effort of transport would have been prohibitive. Building the walls is a slow and skilled process, carefully selecting the blocks of stone and making them fit together. Many of these old walls -and some of them may even date back thousands of years into prehistory- are now in a poor state, because of a lack of maintenance. Others have been repaired using mortar to hold them together. Some have been robbed to build elsewhere, or have been grubbed out and replaced by barbed wire.
Ivy is a serious menace to walls, as are trees that are growing close to or on walls. A recent problem is Buddleja.
The many joints between the stones provide shelter for many insects, spiders, snails and other invertebrates as well as for toads and nesting places for some small birds, such as tits.
Old stone walls can develop a community of plants once weathering has provided a thin soil for their roots, which may run along the joints between stones. The environment is harsh- often catching the full force of rain, but then completely drying out in sun and wind. Many of the plants are ones that you might find in the hotter climes of southern Europe.
Usually, the first things to colonise the stones are lichens.
Small cracks between stones frequently support small ferns, usually where there is old lime mortar.
A fern, spleenwort, Asplenium trichomanes in Bradford.
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Another wall fern, Ceterach, in Broughton Gifford
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Asplenium ruta-muraria, wall rue in Bradford
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A thicker build-up of soil on a wall has allowed a larger fern, Polypodium, to establish itself. A plant of slightly acidic conditions, it grows on limestone because of acid rain leaching lime from its soil.
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he leaves of English stonecrop, Sedum anglicum, are adapted for harsh conditions- rounded to reduce drying and fleshy to store water. Another stonecrop, Sedum acre, with small yellow flowers, is common on the exposed tops of walls too.
See the article on Winsley walls: by John Presland 2008. Dry stone wall: a biological treasure? in the Wiltshire Archaeological & Natural History Magazine, volume 101, pp. 8-17.
More to come!