Islay Geology


Islay has had an extremely varied geological history reflecting the wide range of climates and environments that the Island has been subjected to over many millions of years. Whatever your experience or knowledge of geology there is plenty to see on Islay.

Marine sedimentary rocks such as sandstone, limestone and shale dominate much of Islay. Most sediments have been subjected to slight metamorphism (caused by heat and pressure), changing the original sediments to quartzite, psammite, phyllite and slate.They are of Precambrian age (probably ranging from 1000 to 600 million years old on the Island) and belong to the Dalradian Supergroup and the Colonsay and Bowmore Groups which are both of uncertain stratigraphic position.



One of the most interesting formations seen on the island is the Port Askaig Tillite which forms an important marker formation in the Dalradian, allowing correlation with many areas of Ireland and Scotland. The Tillite was formed during a major Precambrian ice-age.

Not all the rocks on Islay are sedimentary in origin. The southern third of the Rhinns (Western Islay) is composed of igneous rock that has been subjected to many phases of folding and metamorphism.

The Palaeo- proterozoic (1800 million year old) gneisses form the basement onto which the Colonsay Group sediments were deposited. The rock is known as the Rhinns Complex, although it was once thought to be part of the famous Lewisian Gneiss Complex that is seen further north.





There are numerous folds, faults and shear zones on the island. The most important fault on Islay in terms of both the geology and the landscape is the Loch Gruinart Fault. Erosion along this fault has resulted in the formation of the two sea lochs, Gruinart and Indaal.

Structurally the rocks to the southeast of the Loch Gruinart fault are dominated by a major fold known as the Islay Anticline. Most of the structural features on the Island, as with the rest of Scotland, were formed about 500 to 400 million years ago as two major plates of the Earth's crust collided together.

260 million years ago Islay was part of a large desert which later became flooded by a shallow, tropical sea. Sediments that were formed at this time can now be seen as red coloured breccia and sandstone on the southern coast of the Mull of Oa.



About 65 million years ago the west coast of Scotland was dominated by intense volcanic activity during the formation of the Atlantic Ocean. Although there were no volcanoes on Islay numerous associated dykes cross the island from SE to NW. Due to erosion these dykes are now seen on the surface.

During the last Ice Age Islay was completely covered by ice (from 75,000 to 14,000 years ago). The ice eroded and sculpted much of today's landscape and was responsible for the extensive deposits of sand, gravel, till and boulders.

The Islay Natural History Trust Field Centre is a great place to learn about the geology of the Island. As well as an up-to-date display, there are hands on activities (suitable for adults and children) and a well stocked library containing all the latest publications on, and associated with, Islay's geology. So if you are a beginner who is keen to learn more or an expert who wants to check out the latest research on the Island's rocks then visit the Field Centre at Port Charlotte.