Mull and Iona

Mull is one of the larger islands of the Inner Hebrides. Most of the interior of the island is rugged defined only by its natural features and lacking paths or other signs of human influence…this is part of the attraction of Mull.
The southern and western coasts are remote indeed and here the visitor frequently does not meet with any other human beings during the course of a day’s exploration.

Mull contains varied and unusual geological formations and has been a popular destination for generations of geologists. The basal rocks are of late Pre-Cambrian age, more than 550 million years old, overlain by subsequent sedimentary fossil bearing Triassic, Jurassic, Cretaceous and later deposits.
Some of these are of international importance including the “leaf beds” at Ardtun near Bunessan where fossilized leaves can be found from trees that lived in a warm and humid climate. In the Tertiary Period the leaves appear to have fallen, year after year, into a shallow lake and settled to be sandwiched and then fossilized in layers of sand and gravel.
To the west at the tip of the Ross of Mull lies an area of red granite covering more than twenty square miles that is the remains of a large molten magma chamber intruded about 414 million years ago. In the nineteenth century this was extensively quarried, and the stone exported widely throughout Britain. A substantial proportion of the Albert Memorial in London comprises granite from these quarries.

Another unusual mineral are blue sapphires, which have been found on both sides of Loch Scridain, although they are not generally of gem quality.

It is the igneous rocks of the Tertiary period however that provides material of interest to the agate collector. These rocks are to be found over the whole island and are approximately 55 million years old….a fraction of the age of the agate bearing rocks of “Old Red Sandstone” further to the south. The Mull deposits are plateau lava flows surrounding a central caldera or volcanic plug. The flows appear to have been extruded gently in a highly viscous state, flow after flow, and some only a few feet thick. This “trap feature” as it is called is best seen in the Ardmeanach peninsula and the south coast of the Ross of Mull. This feature is caused by differential weathering of the top of the flow and the inner parts of the flow and gives the hillside a stepped appearance.

Agates are only found within a very small proportion of the lava flows, but specimens occur on the Ross of Mull, to the north of Loch Scridain and at Tobermory near the lighthouse. The genesis of Mull agates is something of a mystery because the source of the silica from which they form is unknown. The lava flows in which they are found contain very little silica.
Mull agates are generally rather small, ranging from about 30 to 50mm in most cases, although larger examples do occur. Most are clear and banded chalcedony, and these frequently possess the most exquisite patterns. The agates found here are usually uncracked and show no colour apart from some with a pale honey-coloured tint. The monochrome banding is tight and beautiful, but some agates show very intricate and unusual patterns that are unique among Scottish agates. These agates possess unusual whorly and wave patterning. Another, almost characteristic of the Mull agates is that many of them contain large, often brown, Calcite crystalised inclusions.

The known agate deposits on Mull are localized but it’s a big island and has not probably been fully explored so there may be a lot more interesting areas to be found. Encounters with adders and other wildlife including deer and wild goats are frequent and the hills support numerous orchids and other flowers.

Occasionally in places like this somebody finds some exceptional specimens. This is exactly what a friend of mine, Mike Forster, managed to do while carefully examining the beaches of the south part of the Ross of Mull. After many collecting visits he managed to find these two beautiful specimens below showing the characteristic whorly patterning seen in agates from here.


Another interesting feature of these Mull agates is that they show some fluorescence. The vast majority of them fluoresce yellow under both LW and SW Ultraviolet light (more under LW though)..…..this is most likely due to trace amounts of Calcite within the cryptocrystalline Quartz of the agate structure.


Thanks to Mike Forster for most of these beautiful agates…..


Despite its proximity to Mull, the island of Iona has a very different geological history and the rocks that make up its structure are quite separate from its near neighbour. The country rock comprises formations including Torrridonian sandstone, about one thousand million years old and even earlier Lewissian gneisses from the Pre-Cambrian era, which have been dated at nearly three thousand million years of age, some of the oldest rocks in Britain.

None of the Iona rock formations is known to be agate bearing so the origin of the small, occasionally colourful agates found on some of the southern and western beaches is unknown. There are generally two types of agates found here, brown moss vein agate and green agates occasionally with a trace of red. Where they have come from is unknown but presumably glacial action had a part to play but the source is probably not the Mull as they appear to be different.


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