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The Fascinating Geology of Mull and Iona

This week I am going to take you on a journey back in time to see how and where Mull and Iona’s astonishing geology has formed. I’m enlisting the help of my friend James “Seumas” Westland whose knowledge of the island’s geology is second to none.

Seumas is the island’s go-to man for any geology questions. He also operates Mull Geology Tours, travelling all over the Islands to show people first hand the remarkable history beneath their feet.

From a geology standpoint, the most noteworthy observation is that in the space of two small islands we have some of the most recently formed land in history, as well as some of the world’s oldest rocks.  This age differential would be understandable within an area the size of a large country or a continent, but it is remarkable for two small Hebridean Islands only 58 miles in length.

In short, Mull and Iona are a microcosm of the earth’s geology, with rocks and landscapes dating from as recently as 11,000 years ago, at the end of the last ice age, to rocks that are close to 3 billion years old.  To find these two extremes so close together is an indication of the turbulent geological past of these beautiful islands.

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Quaternary period, between 11,500 years ago and the present day

We start our journey at beautiful Calgary Bay. Most visitors to the Island will have at one point, sat and looked at this beautiful stretch of white beach and blue water. The land around here is from the Quaternary period, between 11,500 years ago and the present day.  If you look up to the right-hand side of the bay to Caliach Point ( the point of the old woman), you see the best examples of a ‘raised beach’ caused by changes in sea level. There are fine examples of scree slopes and a wonderful stretch of machair which backs on to the beach. It holds unique collections of flora which helps halt beach erosion.

12,500 and 11,500 years ago

Moving further back in time we head for Scarisdale and Glen More. The land here was formed by the last glaciers to exist on the Islands between 12,500 and 11,500 years ago. Good examples of glacial deposits (Moraine) and glacial features, like drumlins, can be seen in Glen More while Ross of Mull granite erratics are all over Iona. The P-forms in Scarisdale (glacial grooves in the rocks) are said to be the finest in the UK and are well worth a look.

Our next jump in time takes us to the Palaeogene (aka Tertiary) era, 60 – 55 million years ago. Significant igneous activity (also in Rum, Skye, Ardnamurchan etc.) took place during this period. Lavas, intrusive rocks (dykes, sills, cone sheets etc.) and occasional sedimentary rocks between lava flows all formed part of the landscape. The majority of rocks that are visible today are from this turbulent period. The artun leaf beds are an excellent example of this period and literally hundreds of dikes run across Mull and Iona.

Mesozoic era (Triassic, Jurassic, Cretaceous) 200 million to about 65 million years ago

Moving to the period that Hollywood loves the most, we arrive at the Mesozoic era (Triassic, Jurassic, Cretaceous) 200 million to about 65 million years ago. This period saw the formation of the great sandstone and limestone sheets. The location of these rocks in coastal areas like Gribun, Inch Kenneth, Carsaig, Laggan, Loch Don, and Garmony gives rise to the theory that the Mesozoic rocks probably under lie all of Mull but today are only seen peeping out at the edges.

Palaeozoic (Permian) era, 299  -251 million years ago

The key feature on Mull and Iona dating from the Palaeozoic (Permian) era, 299  -251 million years ago was the formation of dyes or dikes which can be seen all over the islands.

In geological terms, a dike is a sheet of rock that formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma intrudes into a crack then crystallizes as a sheet intrusion, either cutting across layers of rock or through an unlayered mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack, however little evidence of this type exists on these Island. Some of the best examples of magmatic dikes are seen in eastern Iona and on the Ross of Mull.

Palaeozoic (Devonian / Silurian) period

Our next bit of geological time travelling is a serious jump, increasing from 250 million years ago to over 400 million years ago, the Palaeozoic (Devonian / Silurian) period.

Approximately 414 million years ago saw a tumultuous series of geological changes around the west coast of Scotland. This was the time of the great granite intrusion. In particular, the Ross of Mull granite, one of many “Caledonian Granites”  found all over the Highlands, was formed and lavas related to the “Lorne Plateau lavas” near Oban also folded into the landscape.

The best places to see granite is on the Ross of Mull granite fields near Fionnphort and also just offshore Iona.  Evidence of Lavas is also found near Loch Don.

Our final time jump delivers us to some seriously old rocks. The Torridonian Era occurred around 1 billion years ago. Even in geological terms these rocks are old, very old, and are found only on the east side of Iona near St Ronan’s Bay and the Abbey. The age and rareness of these rocks give many people a sense that time has stood still.

On the West side of Iona, near the beautiful Port Ban, are some of the world’s oldest rocks. Dating from the Archaean period, The Lewisian rocks were formed 2,800 – 1,700 million years ago. That is nearly three billion years old in layman’s terms! And given the oldest known rocks on Earth, found in Australia mooted at 4 billion years old means you are sitting above something so old that you would need to visit other solar systems to find its equivalent. Contrastingly the grassy surface you are sitting on is probably Machair, the youngest of all the geological formations! That is the beauty of Iona and what picks it out as a geological wonder.

It is easy to sit in the quiet beauty of Mull and Iona and look at the shape of the land, and the flora and fauna it nurtures, and forget the rocks you are sitting on.

Nowhere else will you find yourself among rocks that might have formed in the geological equivalent of yesterday, or when the earth was still young. I wonder if the age of the land had anything to do with Iona being seen as a home of not only Christianity but of Druidism and Nihilism.

Before you leave these islands, I would advise you to take a stone home with you. Or better still, treat yourself to a piece of Iona or Mull Jewellery set from Martyrs Bay Shop with marble or granite, which is as old as the earth itself. That really would be a lasting investment!