Many thanks to Richard O'Connor for giving me a map of the soils of the area. By comparison with the local geological map, is relatively easy to read: the dark green areas are 'land of little agricultural use', while the light blue areas are 'land suited to the growth of arable crops and grass in rotation' - in other words, the best soils. In between, the pink is better than the mauve.
As can be seen and, I suppose, expected, Ardnamurchan has precious little of the best soils. Perhaps surprisingly, the matter at Sanna is classified as poor soil while other area softer soil, such as the soils overlying the metamorphic rocks at top right, are fully to be expected.
What little good soil there is has been exploited historically by the small clachans that dotted the area in the centuries before the clearances of the 19th century. Some are clustered around the best patches of land. '1' is the small group of clachans which include, from left to right, Ormsaigbeg, Ormsaigmore and Kilchoan. '2' is the three Swordle clachans with, just to the east, Ockle. Finally, '3' is Skinnid - its area of good soil is now one of the Ardnamurchan Estate's silage fields.
What the soil map brings out is how hard it is, both today and in the past, to eke a living from arable farming in this area.