Islay & the Garvellachs: Evidence for ‘Snowball Earth’

Explore Earth’s past climates recorded in the rocks of Islay and the Garvellach islands

Jura Quartzite forming the backing cliffs of the ‘Main Rock Platform’ of the Loch Lomond Re-advance (c. 11,000 years ago) cut into the pre-glacial ‘High Rock Platform’at Baigh an Da Dhoruis on the north coast of Islay. Image: David Webster.

The rocks of Islay include a record of some of the most extreme climatic changes in Earth’s history – major global climate fluctuations that took place some 650 million years ago when glaciers reached the tropics. Glacial deposits indicating a worldwide ice age – or ‘Snowball Earth’ – are sandwiched between rocks that were deposited in much warmer climates. There are four key localities on the eastern part of Islay that record the transition into and out of a ‘Snowball Earth’.

At Beannan Dubh, near Ballgrant, the eroded and heavily fissured surface of the Lossit Limestone Formation is exposed. The limestone contains oolites, or ‘egg stones’; sedimentary rocks made of circular grains of calcium carbonate which commonly form on the bed of shallow seas. It also contains stromatolites, fossilised collections of tiny organisms which would have thrived in a warm temperate climate, suggesting Islay was in the tropics when the limestone formed.

Overlying, and in places infilling, the limestone are a variety of glacial deposits including iron-rich siltstones, sandstones and layers of ‘diamictite’. This is a rare and striking rock; mudstone that contains a variety of pebbles and boulders, many of them granite. It is thought to have formed during an ice age when glaciers, embedded with pieces of continental rock, flowed towards an ocean, perhaps grounding at or close to sea level, before gradually melting and scattering their cargo of glacial debris into the underlying mud. Thick layers of diamictite occur at Port Askaig and along the coast to the south around Fionn Phort; and extend north into the uninhabited Garvellach Islands and through the Scottish Highlands to crop out at other locations, including Schiehallion.

The glacial diamictite deposits are followed by layers of shallow marine sedimentary rocks, including what is believed to be a ‘cap carbonate’. In the end stages of large-scale glaciation, several mechanisms are thought to be responsible for increasing atmospheric CO2 concentrations. This leads to rapid warming, which melts ice and leads to rising seas; the end result is often the accumulation of thick layers of carbonate sedimentary rocks, which are called cap carbonates and are found worldwide in rocks of this age.

At Caol Isla, north of Port Askaig, the rocks overlying the cap carbonate indicate a return to tropical conditions. They include sedimentary rocks such as siltstones and carbonates containing mud cracks and ripple marks. Further north at Bunnahabhain, dolomitic rocks are exposed – these are carbonate rocks composed mainly of the mineral dolomite. Like the Lossit Limestone, these dolomitic rocks contain stromatolites. Dramatic exposures of the stromatolitic layers within the Bunnahabhain Dolomites can be found on the remote North Coast of Islay.

This ‘yo-yoing’ between rocks created in warm shallow seas and those created under glacial conditions that are found on Islay and the Garvellachs have enabled scientists to piece together an intriguing story that provides evidence of a ‘Snowball Earth’.

Contributed by David Webster

Find out more
The Geology of Islay website –
FAIRCHILD, I.J. (1991a): Itinerary II: Topmost Islay Limestone (Appin Group), Port Askaig and Bonahaven Formations (Argyll Group), Port Askaig area, Islay, in LISTER, C.J. (ed.): The Late Precambrian Geology of the Scottish Highlands and Islands. Geologists’ Association Guides, 44, pp. 33–41.
FAIRCHILD, I.J. (1991b): Itinerary III: Bonahaven Formation (Argyll Group), North Coast of Islay, Ibid. pp. 42–52.
SPENCER, A.M. (1971): Late Precambrian glaciation in Scotland. Geological Society, London, Memoirs, 6, pp. 5-102.
WEBSTER, D.J., ANDERTON, R. and SKELTON, A.D. (2015): A Guide to the Geology of Islay. Ringwood Publishing, Glasgow. ISBN 9781901514162

Bedded iron-rich sandstones and dolomitic siltstones (the ‘Disrupted Beds’) in the lower part of the Port Askaig Tillite at Beannan Dubh near Ballygrant, Islay. Image: David Webster.

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