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E-book Clachtoll : An Iron Age Broch Settlement in Assynt, North-west Scotland
Assynt is well known for its complex geology and was the field laboratory of 19th-century geologists Benjamin Peach and John Horne, who pioneered geological mapping techniques and the modern understanding of processes of geological thrust, opening up new possibilities in the study of landscape formation and evolution. Some of the world’s oldest rocks are found in Assynt, gneisses formed over 2600 million years ago, which were in turn overlain by Torridonian c. 1000 Ma, encompassing the sandstones from which Clachtoll broch is constructed. The position of the broch itself is significant in geological terms, being located on Torridonian sandstone but close to the bound-ary between the coastal Torridonian and the gneiss of the western Assynt coast (Fig. 1.2). This has translated into a practical division in agricultural viability in the north-west of the parish, with the inland areas over gneiss typically poorly drained and giving rise to peat-dominated knock-and-lochan landscape while the coastal areas, with more freely draining geology, have given rise to coastal machair and good agricultural land, now mainly used as common grazings. Clachtoll broch therefore occupies a topographic niche common to many Atlantic Iron Age settlements in that it is close to the coast, controlling likely harbours at Clachtoll and Stoer bays and associated with better agricultural land. The geology of the immediate locale was undoubtedly a determining factor in the siting of the settlement, with the sandstone bedrock quarrying easily into tabular blocks, unlike the hard and irregular gneiss of the areas to the north and south of Stoer.The broch itself occupies a rocky crag on a prominent shoulder of rock to the south of Stoer Bay, only 2–3 m above the MHWS, in a particularly exposed position that is prone to the impact of westerly storms (Fig. 1.3). Reconstruction of sea level in later prehistory at Clachtoll is complicated by the complex interplay between isostatic rebound and post-glacial sea level rise, and relative sea level (RSL) models are too coarse to reconstruct this relia-bly, but current models indicate a probable RSL in the first millennium BC that was 1 to 2 m higher than at present (Hamilton et al. 2015). This is unlikely to have been high enough to isolate the broch from the low, level ground to the east during its occupation, but would certainly have provided more immediate access to the sea, albeit that this would presumably also have entailed greater vulnerability to wave damage in winter storms.
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