Although it was once described as "extremely uninteresting" (Macculloch, "A Description of the Western Isles of Scotland" 1819), the geology of Colonsay is now known to be quite exciting. In a nutshell, the Great Glen faultline which runs down from Inverness and along the Firth of Lorne splits northeast of Colonsay. One fault continues towards Donegal, running between Colonsay and the Ross of Mull; whilst the other runs between Colonsay and Jura before splitting Islay in two, via Loch Gruinart and Loch Indaal. Thus Colonsay, Oransay, the Rhinns of Islay and the tiny, northernmost islands of Ireland are geologically connected and are rather different to the surrounding area. The little islands are called Inishtrahull and, together with Colonsay and the rest of the group, broke away from what is now Greenland some 1,800 million years ago; tectonic plate movements led to their being transposed by some 800 miles.
Incidentally, there is a differing and as yet unsubstantiated suggestion that the area outlined above might in fact have no connection with Greenland; by this reasoning, it forms a tiny and quite discrete independent tectonic platelet of its own! Time, as they say, will tell.
As far as we know, the oldest rocks in Colonsay are rather more ancient - in fact, they may be 1,700 billion (i.e. million million) years old; and even the youngest of our rocks predate conventional life on earth, so there are no native fossils to be found locally. Naturally enough, much of the geological structure is only appreciated by specialists, but even the lay person can appreciate the effects of glaciation, can easily identify the raised beaches and can recognise the igneous rocks around Scalasaig. There are also some remarkable dykes, and there is always the chance of a gentle undersea earthquake as the faultlines are eased - usually about strength 3 on the Richter scale and about ten years apart. The most recent episodes took place in early 2012, in the vicinity of Loch Gruinard and Loch Indaal.
Note: The Lovatt family recently (2016) discovered two fossils, evidently brought to Colonsay by glaciation. Apparently they were lycopsids, part of the root structure of the tree-like plants that produced coal seams, dating to the Carboniferous period (350 - 290 m.y. ago).
For further information on local material, please refer to the geological notes attached below.