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Lewisian gneiss on Tiree - Click to enlarge!
Lewisian gneiss

Meta sediments on Tiree - Click to enlarge!
Meta sediments


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History of Tiree

Basalt dyke eroded by sea - Click to enlarge!
'Soft' basalt dyke eroded by sea

Fractures displace rock banding - Click to enlarge!
Fractures displace bands

Quartz crystals - Click to enlarge!
Quartz crystals in banding

Crystals eroded out by sea - Click to enlarge!
Crystals eroded out by sea

Tiree Gneiss - Click to enlarge!

Tiree Marble - Click to enlarge!
Tiree Marble (Coccolite) from Balephetrish Bay

The Ringing Stone - Click to enlarge!
The Ringing Stone - A glacial erratic

Skerryvore Lewisian gneiss reef - Click to enlarge!
Skerryvore on Lewisian gneiss reef

The rocks found on the Isle of Tiree are part of the Lewisian metamorphic suite also found on the Western Isles, Coll, Tiree, Iona, Islay and a mainland Scotland coastal strip of from Cape Wrath to Kyle of Lochalsh. These rocks up to 3000 million years old, are the oldest found in the British Isles and among the oldest on the planet. These Lewisian gneisses were formed at a depth in the earth's crust of many kilometers under great temperature and pressure. They began as granites, sediments and volcanic rocks but billions of years of 'baking' and deformations during numerous crustal mountain building events, have changed the appearance and mineral content of the rocks almost completely.

It is in the nature of such metamorphic basement rocks to be hard, very hard. Originally it was intended to use gneiss from a quarry at Hynish to build the great Skerryvore Lighthouse. However, the Tiree rocks proved too hard to be economically fashioned into blocks and construction switched to using softer Mull granite instead - 'softer' being a relative term in this case. In the above images a basalt dyke instrusion is seen to have been eroded away by the sea while the surrounding Lewisian rocks remain. Almost anywhere else the opposite is observed where basalt instrusions are left standing high as the rocks around them are eroded away.

Tiree's Lewisian rocks have been subjected to the stresses and strains of multiple continental tectonic collisions, breaking up of continents and uplift of mountain chains. Above are images showing how fracturing has displaced the bands within the rocks.

Tiree 'marble' was quarried commercially at Balephetrish in the 19thC. Green and pink pebbles are fairly easy to find at the east end of Balephetrish Bay. Rather than a true marble these are actually coccolite which is a diopside pyroxene based rock. The elemental composition is that of a calcium / magnesium silicate. The colours probably come from traces of chromium and zinc.

In recent times, geologically speaking, great ice sheets covered this part of Scotland and eroded the land, smoothing rock exposures. With the ice came 'erratics' - rocks displaced to places where they don't occur withing the bedrock. The Ringing Stone, on the north coast, is a light grey granodiorite boulder from Rum which is much younger than anything found on Tiree.

The small yellow areas on some of the rocks are lichen, not gold!

Jim Murdoch BSc, MSc, MBCS, CITP, CEng, FGS, FRAS

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