The Islands of Mull and Iona have some of the most fascinating and complex geology in the world.
Much of the islands are volcanic in origin but the most recent major geological agent to shape the topography has been the last Ice Age when glaciers carved up the landscape. Mull & Iona are truly a “land of fire and ice”
The various geological layers relate to distinct time periods. The very oldest rocks, dated at over 2000 million years, are found on the Isle of Iona. These are ancient rocks, highly altered gneisses of what is known as the Lewisian. Rocks of similar age are found in Tiree, Coll, the North West Highlands and the Western Isles.
Moving on, the next main group of rocks that we find are part of what is known as the Moine Supergroup. These are ancient folded, metamorphosed sediments, around 1000 million years old, which form the main mass of the Northern Highlands and probably underlie most of Mull. We see these ancient rocks peeping out on the shore near Ardalanish, where the minerals garnet, kyanite, andalusite and tourmaline can be found.
The Moine rocks are also found near Gribun. The main rock type is psammite, a metamorphosed sandstone which makes up most of the Wilderness, a dramatic landscape of cliffs, sea stacks and natural arches, shot through with yawning chasms and fissures.
In the very south west of Mull we find the pink Ross of Mull Granite. This has been dated at about 420 million years old. It is very different from the metamorphic rocks of the Moine series and has been extensively quarried in the past. It gives rise to a very gentle rounded topography. Large boulders of this rock, known as erratics, are found in Iona where they have been deposited by glaciers during the last Ice Age.
The next important group of rocks are of Mesozoic age. The Mesozoic Era is made up of three periods: the Triassic, the Jurassic and the Cretaceous. All three are found in various parts of Mull. The Mesozoic rocks are rich in marine fossils such as ammonites and belemnites. The rocks consist of sandstones, shales and limestones. Carsaig is probably the best place to see these Mesozoic rocks, rich in fossils.
The final sequence of rocks to be found is the Paleogene lavas. About 60 million years ago, the whole west coast was tectonically very active and volcanoes poured out huge amounts of basaltic lava. Much of the lava is columnar and nowhere is this structure better seen than in the Island of Staffa which lies off the west coast of Mull. Staffa is one of the natural wonders of the world. Columnar basalt is also well seen on the south coast of Ulva, which is more accessible than Staffa in poor weather conditions.
Much of the basaltic lava has flowed considerable distances and built up great tiered, terraced hills which have a very distinctive “stepped” appearance. The Ardmeanach peninsula, well seen from Bunessan shows this effect beautifully as do the low hills of North Mull
The whole of Mull is criss-crossed by dykes. These are basically fissures which have filled with molten rock and when the processes of erosion take place, a wall like structure can be left. They occur in profusion at Loch Na Keal, but the best example of an easily seen dyke is the one near Calgary Pier where it has been used as part of the wall of a boathouse!
The Ice Age
Relatively recent in geological terms, about 15000 years ago, Mull was covered in glaciers which carved the rocks and gave us roughly the terrain we see today. Evidence is abundant for this. Glacial erratic boulders of granite are found in Iona, scouring of the rocks by ice is clear in many places. On the shore of Loch na Keal, a feature called “P-Forms” can be seen – channels cut in the rock by glaciers. Further evidence of glaciation is afforded by the presence of hundreds of little mounds called drumlins – these are clearly seen in Glen More and represent moraine, or detritus left behind the glaciers.