THE TRESHNISH ISLES - GEOLOGY AND TOPOGRAPHY
The geology of the Hebrides is complex, but one highly significant feature is the existence of the
remains of several extensive Tertiary volcanic centres, one of which encompasses Skye and St.
Kilda, and another seen in much of Mull and its outliers. This geology is characterised by granite,
gabbro and other volcanic rocks, and the columnar and other forms of basalt so characteristic of
Mull, Staffa and the Treshnish are due to the same cause. Volcanic activity has been compounded
by earthquakes, and modified subsequently by extensive glaciation.
Large areas of basalt resulting from the lava flows form parts of Mull and other southern islands,
but especially distinctive is the stepped basalt landform shown so well in the terraces on Lunga,
western Mull and elsewhere, often associated with the vertical jointing caused by slow cooling of
the molten rock.
The other factor which has had massive impact on the landforms of the Treshnish and similar sites
is the sea, taking effect both through wind and wave action. Though far less exposed than the
Atlantic shores of the Outer Hebrides, there is a ten-mile stretch of open sea before the marginally
effective shelter of Tiree to the west.
The coastal landscape is marked by massive boulder cliff collapses, extensive erosion platforms,
exposed west and southwest cliff faces, no beaches except steep boulder beaches, in constant
change due to strong tidal flows. Cave creation and extension occurs – for example the Dorlinn
gap which is gradually meeting in the middle of Lunga, and the through cave tunnel in the south
part of the island. The succession of steeply stepped plateaux levels, especially on Lunga, gives
dramatic scenery but a greener, more hospitable island than many.
The resulting soils and land forms are fertile and well drained, though highly exposed to wind and
spray. Their limited use by man has, nonetheless, left visible effects in the remaining rock walls,
middens and cultivation patches. For long, regular cattle grazing, aided by a vast rabbit population,
kept the sward in good order, but the cattle’s removal has led to serious deterioration, affecting the
welfare of Puffins, Shearwaters and Rabbits. Visitors are now a serious cause of erosion.