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Snails in unsuitable places

October 1, 2010

Cailbeal Dubhgail (Dugall's Chapel), Tobha Mòr, South Uist

I was recently in the Outer Hebrides, visiting the Uists and presenting a paper on snails from Hebridean archaeological sites as part of the Hebridean Archaeological Forum. While there, we visited a number of interesting archaeological sites, some I’m sad to say at real risk of eroding into the sea. As my PhD research is based on snails in the Hebrides, I was sure to keep an eye on likely habitats.

The main land snail habitat on the islands is the machair, a strip of grass-covered wind-blown sand which runs up the west coast (and produces incredibly tasty potatoes). Since about the late Bronze Age (I’m hoping to be able to determine a more accurate date), this has been dominated by two species of snail – Cochlicella acuta and Helicella itala, although numerous other species do live there. The machair is calcareous, being made up of shell sand, so it provides an ideal substrate on which snails can flourish.

Less suitable for snails are the blacklands, the acidic peat that lies between the machair and the mountains to the east.  Pockets of suitable habitat do exist however, and one of these is mortar on buildings. Most of the old walls and buildings are of drystone construction, and so don’t provide the calcium snails need to build their shells. Churches and chapels are different, however. At the conference, Mark Thacker, of York University, presented a thought-provoking paper about the use of shell mortar in religious buildings. The use of mortar in non-Romanised parts of Britain arrives with the Christian church, and represents the creation of distinctive buildings set apart from secular houses by the use of highly visible white mortar. An accidental consequence of this is that suitable habitats for snails are created in otherwise relatively inhospitable landscapes.

I had the opportunity to look at two groups of mortared chapels. At Tobha Mòr on South Uist, I saw Helicella itala and Cornu aspersum living on the walls. On North Uist, at Teampull na Trionaid, there were many examples of Clausilia bidentata (a typical wall-dweller) and Lauria cylindracea, as well as individuals of Cornu aspersum and Oxychilus alliarius.

Cornu aspersum at Teampull na Trionaid

Cornu aspersum at Teampull na Trionaid

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