The Isle of Iona: An island of contrasting rock, colour and light

An island of superb geological contrasts where multiple rock types display a vast range of colours and textures, all against a backdrop of white sand, green machair and turquoise sea

The famous “Bay at the Back of the Ocean” on the west side of Iona. A storm beach of brightly coloured pebbles, mainly of gneiss, micro-granite and amphibolite. Image: James Westland.

There are two main rock types represented in Iona. On the west side of the island, the rocks are mainly Lewisian Gneiss (over 2000 million years old). On the east side, altered sedimentary rocks, known as the ‘Iona Group’, which are about 1000 million years old and have affinities with the Torridonian rocks found elsewhere in the North-West Highlands.

The gneiss takes on many forms, and is derived from both sedimentary and igneous rocks. Coarse-grained pegmatites – rock containing large, often interlocking crystals – can be found in a few places. Several bands of marble crop out in the gneiss and one of these was commercially worked in the past, the remains of which can be seen in an inlet on the south-east corner of the island.
If you look closely at the Lewisian Gneiss, and at the younger ‘Iona Group’ rocks on the east side of Iona, you might notice bright red, sheet-like veins of another type of rock – this is a micro-granite that has intruded into the older rocks of the area, and is associated with the Ross of Mull granite in terms of age.

Igneous lamprophyre rock of Permian age can be found in the form of numerous dykes – straight lines of rock cutting through existing rock layers on the east side of the island. These dykes commonly weather out to form channels, being softer than the rocks into which they have been intruded, leaving a strange series of lines in the landscape that could be mistaken as being deliberately carved out by humans.

On the island’s west side there is a large expanse of ‘machair’ – grass-covered, wind-blown shell sand of very recent (post-glacial) age. Evidence of the last ice age is everywhere in the form of erratic boulders formed mainly of Ross of Mull granite which has been transported by ice from the east – these rocks now rest on top of the older rocks where the ice deposited them.

One of the great delights of Iona are its pebble beaches; the most famous being ‘Columba’s Bay’ in the south east, where the brightly-coloured pebbles, freshly washed by the waves, provide a visual treat for beachcombers. In amongst these pebbles, you might be lucky enough to find small pieces of the yellowish-green variety of ‘Iona Marble’, which is much prized for making jewellery.

Text contributed by James Westland

Find out more
Mull and Iona Geology: http://mullgeology.net/

Mull and Iona – a landscape fashioned by geology by John Merritt and Graham Leslie provides a great introduction to the geology of the wider area. Download from https://www.scottishgeology.com/find-out-more/publications/#northeast

Iona Machair. Holocene wind blown sand, covered in grass overlying Lewisian Gneiss of Archaean age. An extreme contrast in ages. Image: James Westland.

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