On the Rocks


The rocks that we see on Islay give us access to a mind-blowing and grandiose history of the planet. The timescales we’re dealing with are the last 1.8 billion years – reaching halfway back to the very formation of the earth. There is passing evidence of oceans that pre-date the Atlantic, past glaciations, unrecognisable continents, signs of the beginning of life on earth, and the vestiges of the much more recent ice age just a few thousand years ago.

We’re interested in the geology because of its direct bearing on human-era applications which are our bread and butter. Agriculture, because of the soils that the bedrock gives us in which malting barley is grown. And whisky-making, because of what the geology means for the water which we use.

Islay is not like a volcanic island that was all formed out of one type of rock, or at one point in time. Several rock types and aeons collide here.

geological map of islay

Simplified map – British Geological Society

Islay is surrounded by fault-lines. One fault-line runs through the middle of Loch Indaal. David Webster, who had a career in the oil industry before making retirement plans to write a book of geological walks and build a house on Islay just behind the distillery, remarks, “If they drained the loch, they’d probably find dinosaur fossils down there off Port Charlotte! The scallop dredgers have turned up ammonites from the same era…”

David Webster

“They’d probably find dinosaur fossils down there off Port Charlotte!”

The Rocks of the Rhinns

The most famous of the Rhinns’ rocks, down where we are, are ancient gneisses. They were formed as part of a volcanic arc connecting Labrador, Greenland and Scandinavia, with Islay somewhere in the
middle, 1,800 million years ago. They are known throughout the world as the Rhinns Complex.

These once igneous rocks, formed deep beneath volcanoes, have been metamorphosed, under intense pressure and heat, while buried maybe 50 kilometres down in the earth’s crust. These extreme conditions cause the crystals in the rocks (which you’ll see as speckles in rocks like granite) to reform into visible bands.

The Rhinns is criss-crossed with fault-lines, which has led to the exposure of these rocks which normally lie deep within the earth’s crust. On the shore at Bruichladdich, just outside the distillery, there are two types of the metamorphosed igneous rocks lying around for all to see. A pink-hued ex-granite so coloured because of the potassium feldspar in its mineral mix, and a green ex-basalt containing hornblendes. Green and pink pebbles are very common on the beaches of the Rhinns.

Pink rocks rich in potassium feldspar

The rocks have, over millennia, been eroded into sands which have in their turn been compressed into rocks – sandstones, called the Colonsay Group – some of which have also then undergone metamorphism, into meta-sandstones. The meta-sandstone rocks in the Rhinns are also, in the scheme of things, relatively old, about 800 million years, pre-dating the explosion of life on earth.

What’s cool in the Rhinns is that there are places where the two types of rocks, the Gneisses and the Sandstones, are thrust together cheek by jowl. And sometimes, where the junction is on the coast, the waves have cut a cleft along the fault line – like at Kilchiaran – giving us a visible ‘billion-year’ age gap.

geology Kilchiaran Islay

Overview of the rest of Islay

In the north of Islay, around Bunnahabhain and Bolsa, there are 650 million- year- old limestones that contain fossilised mounds of algae that were earth’s first multi-celled lifeforms. These cyanobacteria lived and multiplied in a warm sea, the Iapetus, which existed prior to the arrangement of landmasses and oceans that we know today. The algae sometimes grew in large colonies, called stromatolites, and can be “as big as a whisky barrel” according to David. They were common in these ancient seas because,“The grazers who would normally eat algae, such as whelks and snails, hadn’t been invented yet. Islay is the best place in the British Isles to see these stromatolites,” he adds.

Thick beds of dark grey limestone, slightly older than that at the coast, occur around Ballygrant. That is where the lime for improving the fertility of Islay’s fields is quarried and ground. It is full of dead plankton, apparently. The pH of limestone is high, alkali, containing calcium carbonate.  The pH of other rocks is low, acidic, as they contain more silica.

About 550 million years ago, England and Scotland were on different super-continents – separated by the Iapetus Ocean. About 470 million years ago, when there was marine life and just the beginnings of plant-life on land, the Iapetus ocean closed. This created the Caledonian mountains. You can see the drama written into the rocks in the Port Ellen area, where the rocks of different ages and origins have all been tilted and metamorphosed by the forces involved.

Then 55 million years ago, just after the extinction of the dinosaurs, the Atlantic ocean was formed along where the earth’s crust tore between North America and Eurasia. The northern shore of Islay is the place to see evidence of that event, where there are  “enormous vertical sheets of frozen magma, called dykes.”

Rocks originally deposited on the margins of the Iapetus ocean


Glaciers, massive sheets of slow moving ice that have scraped and moved and moulded the landscape, have had some effects in Islay that are of interest to our theme today.

As they melted, they left behind a mixture of of rocky mud known as boulder clay over much of the island. These give us the clay soils that often bog down farming equipment. Lower lying areas were then covered by peat bogs.

As the climate warmed at the end of the last ice age, still long before civilisation as we know it came about, sea levels were several metres higher than they are today. This has left us with rocky beaches on cliffs or inland in the “wrong” places.

It is possible that Loch Gorm, a few miles west of the distillery, is a “kettle hole” where part of a glacier was trapped with rocks and mud on top of it, later melting into a “boggy hollow” (David’s words). The fields surrounding the Loch at Ballinaby and Rockside are underlain by gravels and sands deposited by rivers coming from the melting glaciers, giving them good drainage and better soils for growing barley for whisky-making.

Barley growing out of a raised beach at Blackrock, Islay

The water table in Islay is quite high, because of our wet climate, David explains, and drainage is certainly a feature of many an Islay Barley farmer’s plan. Whatever mechanical and chemical interventions the farmers can make in order to overcome the pre-existing conditions, the picture remains that Islay’s geological history is so complicated, that the soils are a sort of pot-luck. Even within the one farm, fields can vary.

Andrew Jones, whose farm is 6 miles west of the distillery on the Atlantic coast, shares some details of his land. “Field-wise at Coull Farm, it’s extremely difficult, we’ve got all the evils. We’ve got sand, we’ve got peat, we’ve got rock. Rock is hard on machines and the sand is good growing soil but in the wrong type of conditions, it’s apt to blow, you’ve got drifting. You’ve got all sorts of challenges.”

In the case of Blackrock and other farms at the head of Loch Indaal, barley is sown into fields on top of a raised beach. The stones here might look like a hinderance but they actually promote drainage and water percolation, as well as holding the heat under the ripening crop. Donald McCormick took the earliest Islay malting barley harvest on record from one such field this year, on August 10th (see photo above).

Duncan at our production water holding loch An Torran

Islay Water

Rainwater is slightly acidic, from picking up CO2 in the atmosphere. If the rainwater only gets as far as a hillside loch, it remains “soft”. Water can get through permeable rocks and as it does, it picks up minerals. Particularly if it flows through limestone it will be “hard”, that is less acidic. River water has very different chemical composition to rain water, as more substances and ions have had chance to dissolve in it, according to the Open University.

All these types of water are used in the production of whisky on Islay by the various distilleries. Some of the waters used in mashing have had their mineral contents analysed by a team from Stockholm University.  The study of how the water in the mash goes on to effect the whisky is ongoing. Generally harder waters slow down fermentation and can lead to more complex alcohols and other esters being produced.

Because we are one of only two distilleries who bottle all our whisky on Islay, there is an additional geological feature in play in our finished products. A spring.

The Octomore Spring

Our bottling water comes from a local spring. It bubbles out of the complicated and mind-bogglingly ancient rock tapestry of the Rhinns. The spring’s local name is Tobar an Uisge Fhìor – “the well of the true water”. It is on Octomore Farm’s land. The Octomore spring water has also been analysed by the University of Stockholm. It is, “Nothing like the surface water,” confirms David. It was found to contain silica at a concentration of 17 parts per million. This is unusually high and unlike any other water on the island.

The well house for Octomore spring, Tobar an Uisge Fhìor

David explains that all rocks are rich in silicates, the earth’s crust, “is oozing with silica,” but it does not easily dissolve in water. There are some waters from volcanic areas around the world that will contain much higher concentrations of silica (one of them is even marketed as “earth’s finest water”).  For water to contain any silica at all, it must have acquired it at temperatures that we never see on the surface, and over a long period of time. Meaning that it has had to travel a long way, from very deep, so it is likely to be quite old.

The fact that the water rises to the surface at all, is what David attributes to “a geological accident.”

The spring is situated by the banks of a stream at the bottom of a glen, Gleann Mohr, above the village of Port Charlotte and used to be the village’s water supply. It’s likely that river action has eroded the cover of glacial material there and exposed one of the fault-lines that criss-cross the Rhinns. These faults are the conduits that have allowed the water to come up from its hidden depths.

And from there, it goes into all bottling-strength whiskies (and the gin) that we make. Carrying a piece of Islay’s geological inheritance to every corner of this modern, whisky-drinking world. Savour it. It took hundreds of millions of years to make.

Further Reading 

A Guide to the Geology of Islay: Webster, Anderton and Skelton. (2017): Ringwood Publishing Glasgow https://islaygeology.org

David’s video presentation for the Royal Philosophical Society of Glasgow from earlier this summer:  https://youtu.be/bH77Ss1d1e0

With thanks to David Webster for the pics and the facts.

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