The fascinating geology of Colonsay

May 15, 2017

The islands of Colonsay and Oronsay, which are connected by a narrow strand that becomes dry at low water, are among the most fertile of the Hebrides. They lie at the entrance to the Firth of Lorn, with Mull to the north, Islay to the south and Jura to the east; on the west, however, the islands are exposed to the full force of the Atlantic gales.  

The islands are formed of rocks that are described as Lower Torridonian in age, comprising limestone, phyllites, mudstones, flags, grits and conglomerates. Plutonic intrusive rocks outcrop at Scalasaig, and to the north there are dykes and sills of lamprophyre and dolerite.

Colonsay and Oronsay are at the south-west end of the Great Glen Fault, with the main line running to the east. The conspicuous ridging, which plays an important role in the topography of Colonsay, results from fault lines.

The islands were greatly affected during the glacial period, when ice, radiating outwards from a centre on Rannoch Moor on Scotland’s mainland, flowed south-westwards over them.

Remarkable, too, are the traces of pre-glacial marine erosion and the lines of the pre-glacial sea-cliffs, most notably between Scalasaig and Machrins, at Uragaig, and to the south of Balnahard. Traces of the so-called “50 foot” raised beach may be seen at Uragaig and Balnahard, and the “100 foot” beach may be seen at Kilchattan.

The marinecut platforms and raised beaches frequently form level areas of well-drained land that have attracted settlement and cultivation from the earliest time to the present day.

The Atlantic coastline has led to the development of sand-dune and machair landscapes that are more reminiscent of the Outer Hebrides than of the rest of Argyll.