Islay

4th June 2018

I'm back from staying with my uncle on the Isle of Islay. This is the most southerly large island of the Inner Hebrides. It looks northwards to Colonsay, eastwards to Jura, southwards to Ulster and westwards to nowhere - or rather into the vastness of the Atlantic Ocean and beyond, to the chilly coast of Labrador. For better or worse, its climate and weather are intimately connected with the ocean.

A misty, eventide view looking south from Ben Cladville, with Donegal in the distance

Spacious Islay - open to the elements. It's a marked contrast to my cramped Suffolk homeland, with its woods, hedged fields and clustered villages. Houses on Islay are white-painted and built of stone and slate; they are strung in rows through 19th century settlement towns such as Bowmore and Port Charlotte, or scattered in farmsteads with strange, hybrid Gaelic, Norse and English names. Many of them have fine views over rough pastureland or boggy moorland, greenish-grey, brown and dotted with rocky outcrops and boulders, and a backdrop of mountains. Many of them have fine views over the sea, which is never far away with its stormy grey or shades of passionate blue and green. The inlets of Loch Indaal and Loch Gruinart almost divide the island in two.

View across Loch Indaal towards the distant Paps of Jura.</ font>

My uncle lives in the Rinns of Islay, a broad peninsula ten miles long at the south-western end of the island. The family farm is perched on a south-facing hillside with spectacular panoramic views to Portnahaven and a scattering of islets. The place is buffeted by wind and rain for many months of the year. The land includes over half a mile of sea cliffs, with caves, precipices and a natural arch, and even a prehistoric dun (promontory fort) and a cleit (burial mound). There are choughs and ravens, peregrines and curlews, stonechats and skylarks. Starlings bubble and squeak from the chimney pots. Sparrows and siskins jostle for space at the bird table.

View over Cladville and the south-western tip of the Rinns.

Rinns rock

The Rinns is founded on very ancient bedrock: gneiss dated to about 1.8 billion years old (by way of comparison the Earth is 4.6 billion years old). Chemical analysis tells us the gneiss originated in the roots of a volcanic mountain range[1], perhaps 20 miles down. Tectonic processes over eons of time have since brought it to the surface. The rocks of the Rinns Complex, as it is called, lie all round Cladville and Portnahaven in the form of the pinkish-coloured gneiss and a greenish-grey amphibolite. The one looks rather like frozen sausage meat; the other like frozen salt and pepper. Both are very hard, and were created through metamorphism of the deep crustal rocks syenite and gabbro respectively. 

A boulder of syenitic gneiss
with a quartz vein and lichen, Claddach

Sea-smoothed gabbroic amphibolite, Lossit

These rocks are the oldest thing I have ever contemplated. They originated in the late Palaeoproterozoic Era of the Precambrian Supereon. They are thought to have formed deep within a volcanic arc where the crust of the Columbia Supercontinent was being subducted.  Today - risen from the depths - they form the cliffs and underlie the boggy moorland of the Rinns.

These rocks push my understanding into a new dimension: they take me as far back in time as I can go in British geology. The only life-forms on Earth then were bacteria and algae.[2] They belong to a world dominated by physical processes in which micro-organisms had a precarious foothold. Today, 1.8 billion years later, the same natural processes continue but micro-organisms are everywhere, forming the foundations for the pyramid of complex life which surrounds me, and of which I am a tiny part. Apparently I may well have more micro-organisms inside me than there are cells in my body.[3] Elements of the Palaeoproterozoic world survive, and not just on Islay.


Snowball Earth

A week ago today I took a bus trip up to Port Askaig, crossing the island diagonally. On the way, I stopped at the Islay Natural History Trust centre in Port Charlotte, where there are displays about geology as well as wildlife. It was an opportunity to get a better overview of local Earth history. Among the specimens on display were samples of the Port Askaig tillite.

Tillite from Port an t-Seilich, near Port Askaig.

Seen here, the specimen has a grey, sandy mudstone matrix containing pebbles of granite. The mudstone is a lithified example of glacial till (hence 'tillite') deposited in very shallow sea water, with its matrix derived from the erosion of shales and granites[4]. The pebbles dropped into it either from melting ice floes or - more likely - melt-out from the grounded base of a wasting ice sheet.[5]

The tillite is exposed along the coast at Port Askaig and also in the road cutting next to the ferry terminal. On arrival, I knew only had two hours before my bus returned so decided to focus on the cutting, which has recently been extended. I soon found examples of granite and other pebbles embedded in mudstone.

A granite pebble in mudstone matrix.

This tillite has a special place in geology. It was first recognised as a glacial rock formation as far back as the 1870s, and attributed to the early Cambrian.[6] Where does it fit in the geological timescale today? It is classified lithostratigraphically as part of the Islay Subgroup within the Dalradian Supergroup, and deposited during the Cryogenian Period of the Neoproterozoic Era of the Precambrian. Evidence from remnant magnetism and carbonate chemistry in these rocks suggests that Britain lay close to the Equator at this time, however the fact that we have clear evidence for glaciation at this very low latitude has suggested that the Earth may have undergone periods of very extensive ice cover. This has given rise to the 'Snowball Earth' concept, which envisaged prolonged periods in the Neoproterozoic when the Earth was frozen as far south as the Equator. The Port Askaig tillite is most likely attributable to the Sturtian Glaciation, part of which has recently been dated to 716.5 million years ago.[7]. Back in Victorian times, Thomson speculated about the origin of the granite clasts. He found he couldn't relate their mineralogy to any extant granites in Scotland:

‘If ... we compare the embedded boulders of granite with the granites found in situ throughout the Highlands, we feel the necessity of tracing them to another source, and hope we do not overstep the bounds of prudent speculation in suggesting that those erratics are the reassorted materials of some great Northern Continent that has yielded to the ceaseless gnawing tooth of time, leaving scattered fragments as wreckage of its former greatness, and that the material of which the mass is composed have in time, deeper than we have hitherto suspected, been transported by the agency of ice.’

We now know that the mudstone matrix was derived from some now-vanished source rocks somewhere south-east of Islay.[4] The granites may well have been derived from rocks related to the syenites of the Rinns Complex, probably an eroding part of the 'igneous province' of tectonically mobile crust which then spanned Scandinavia, Greenland and north-west Canada as part of the Neoproterozoic Rodinia Supercontinent.[8] I touch my hat to Thomson for his scientific insight.

Image courtesy Fitches et al, 1996.

An ice age legacy

Jumping forward 716,250,000 years in time, Islay was covered with ice at the height of the last cold glacial period, the Devensian Stage of the late Pleistocene.  The limits of the ice sheet are thought to have lain many miles to westward at this time.

Devensian ice limits about 23,000 years ago.
Image courtesy Clark et al, 2012, fig.18.

The Devensian, and presumably earlier glaciations, have left traces everywhere on Islay. There is ice-scratched and -scoured bedrock and other glacially-sculpted topography; there are eskers and moraines, sub-glacial meltwater channels; layers of till and outwash gravel. The general direction of ice flow was towards the Atlantic. The till is unevenly distributed, and underlies the most fertile parts of Islay. My cousin William has noticed that his best farmland lies to the south-west side of Ben Cladville, and speculates that the till was deposited more thickly in the lee of the hill as the ice flowed over it. This seems very plausible to me.

Cnoc Bhi Bhuirn, a glacially-sculpted rocky knoll in the classic shape of
a roche moutonée, The direction of ice flow was from the right (north-east).
Ben Cladville is in the far distance.

A glacially-scoured outcrop of gneiss bedrock, with accentuated jointing.  

An erratic boulder of dolerite in glacial till at Claddach.
Dolerite is found on Islay as intrusive igneous dykes of early Tertiary age.

A distinctive ridge of glacial moraine (the 'Blackrock Moraine') at the head of Loch Indaal.
It is thought to have developed in a retreat phase of the last ice sheet.[10]

Another legacy of the last ice age is a suite of raised beaches on Islay. They were formed at a time when the land was depressed by its overburden of ice, but were raised up as the land rebounded after the ice sheets melted and retreated. These are well displayed as a series of planed surfaces in the coastal landscape around Portnahaven.

Raised beaches on two levels at Orsay island, Portnahaven:
c.+ 15 m OD near the lighthouse and c.+10 m OD at St Oran's Chapel (right).

A raised beach at c. +15 m OD behind the houses at Portnahaven.
(Snoozing grey seals in the foreground.)

Coastal deposits on a raised beach at Portnahaven, at the +10 m OD level.

After the ice

The ice sheets have retreated, but seemingly only just. The blanket of meadowland, bog and heather can scarcely cover the the bare bones of the recently glaciated landscape. I say recently, but the ice sheets vanished from Islay between 16,000 and 17,000 years ago.[9]

There is a pass called Bealach Froige on the north side of Ben Cladville. It looks like a glacial overspill channel through which meltwater once flowed westwards. It has a gently sloping long profile and a level, boggy floor which terminates abruptly in a steep declivity, breaking down to a narrow, rocky inlet called Port Froige. The level floor may be the remnant of a raised beach or - more likely - a moraine-dammed lake, as there appears to be a bar of glacial till defining the steep break of slope and marking the still-stand of a small retreating lobe of ice. 

Bealach Froige, a likely glacial meltwater channel - view looking west.

Reddish-brown till exposed at the seaward lip of Bealach Froige

In the 1980s the valley's sediments were cored to a depth of 7.5 metres and analysed for fossil pollen. The results have provided information about the environmental history of Islay in the early post-glacial period.[11]

Far from being the treeless moorland we see in the Rinns today, the pollen record shows that the Cladville area was once forested with birch, Scots pine, elm, oak, alder, hazel and willow. The Bealach Froige profile is undated, but comparison with dated profiles from nearby Loch a' Bhogaidh suggests the expansion of forest took place about 9,000 years ago.[11] There is oak and hazel woodland elsewhere in sheltered parts of Islay, no doubt directly descended from the first colonising trees.

Oak woodland at Kildalton in south-east Islay.

The earliest evidence of human life on Islay after the ice sheets retreated is flint tools of the Ahrensburgian industry, product of late glacial and early post-glacial horse and reindeer hunters. Ahrensburgian-type chipped stone tools have been found beneath Mesolithic layers at Port an t-Seilich.[12] The excavators speculated as to whether these people may have been seal hunters.

The Mesolithic site of Bolsay Farm, close to Loch a' Bhogaidh, has yielded large quantities of microlithic flintwork,[12] and is dated to about 7,930 years BP. David has found a single worked flint flake on his cliff top, so perhaps there is a prehistoric campsite under the peat close by. It could be Mesolithic or equally Neolithic, as there is evidence of Neolithic activity at Loch a' Bhogaidh in the form of stone tools and pollen profiles that indicate forest clearance.[13]

For peat's sake

Peat is found everywhere where there is poorly-drained land on Islay. Initially formed over wet patches and hollows, it spread and coalesced to form a blanket over areas of level moorland. Dead plant debris marinades in a deoxygenated stew of its own decay, becomes compacted, turns into layers of peat. It has been forming across Islay for millennia, but received a boost 4,000 years ago in the early Bronze Age when the climate became wetter and forest clearance was well advanced.[13]

I first came to Islay in the summer of 1968, and remember seeing drying stacks of peat dotted across the landscape near Porthahaven. It was a picturesque sight. Today, fifty years on, very few people continue the practice; David tells me that the struggle to cut the stuff, dry it and cart it away are now beyond most people's patience and endurance. The trenches near Portnahaven are now healing over. Elsewhere, it seems the only large-scale peat cutting still carried out on Islay is destined for the whisky distilleries. They rely on peat smoke to lend its distinctive aroma to the drying barley used in the malting process. The barley itself was originally grown on local areas of loamy soil, though today most of it is imported.

Vegetated former peat cutting scars near Claddach, Portnahaven.

Some small-scale peat cutting near Airigh Sgallaidh, north-east of Ben Cladville.

Peat cutting at Duich Lots. Perhaps the peat is going to one of the nearby distilleries 
at Ardbeg, Bowmore, Lagavulin, Laphroaig or Port Ellen.

As compacted plant remains, peat contains the layered history of the successive plant communities of which it was formed. Its twigs, leaves and pollen are an immediate archive of plant history in the landscape. The deepest peat contains the longest archive. Yes - this means that peat on the fire is part of an ancient landscape library going up in smoke.

But we needn't worry too much, as each volume - each shelf even - is pretty much the same for a given patch of landscape: small variations on a theme in a repetitive music score; a few changed words in otherwise identical paragraphs. Only when we have research questions to answer, as at Bealach Froige and Loch a' Bhogaidh, is it worth the time and trouble to play the score or decipher the text.

Life goes on, oblivious, over and within the Holocene peat bogs of Islay. They hum with insects of all kinds, from infuriating midges to serene damselflies; from gadding cleggs to bumbling bees. Bacterial and fungal action as ever carries out its dark alchemy. Cotton grass flurries in the Atlantic wind which almost constantly buffets the island. Sphagnum moss absorbs water, swells and dies; provides nutrients for its neighbours and descendants. Sundews clasp flies and draw them down into dissolving liquors.

Out on a bog near Airigh Sgallaidh a cuckoo calls incessantly from the echoing walls of a forestry plantation. Palaeoproterozoic rocks continue their unimaginably long journey to becoming grains of sand. It is a Monday morning, and everything has its special place in the world's story.

Blanket bog on Islay, springtime. 

Variations on a theme of peat.

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REFERENCES

1. Muir, RJ. The Precambrian Basement and Related Rocks of the Southern Inner Hebrides, Scotland. PhD thesis, University of Wales, Aberystwyth, 1990.
2. Conway Morris, S.. The Early Evolution of Life. In: Brown, GC, Hawkesworth, CJ & Wilson, RCL (eds). Understanding the Earth. Cambridge University Press, 1992.
3. Sender, R, Fuchs, S, & Milo, R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol 14(8), 2016. Online at https://doi.org/10.1371/journal.pbio.1002533. [Accessed June 2018]
4. Panahi, A & Young, GM. A geochemical investigation into the provenance of the Neoproterozoic Port Askaig Tillite, Dalradian Supergroup, western Scotland. Precambrian Research, Vol.85 (1–2), 1997.
5. Spencer, AM. Late Pre-Cambrian glaciation in Scotland. Geological Society of London Memoir, no. 6, 1971.
6. Thomson, J. On the stratified rocks of Islay. Report of the 41st Meeting of the British Association for the Advancement of Science, Edinburgh, John Murray, London, 1871.
7. Macdonald, FA et al. Calibrating the Cryogenian. Science, 327 (5970), 2010.
8. Fitches, WR et al. Provenance of late Proterozoic Dalradian tillite clasts, Inner Hebrides, Scotland. In: In: Brewer, TS (ed.) (1996). Precambrian Crustal Evolution in the North Atlantic Region. Geological Society Special Publication No. 112, 1996. 
9. Clark, CD et al. Pattern and timing of retreat of the last British-Irish Ice Sheet. Quaternary Science Reviews Vol. 44, 2012.
10. Peacock, JD. Late Devensian palaeoenvironmental changes in the sea area adjacent to Islay, SW Scotland: implications for the deglacial history of the island. Scottish Journal of Geology, 44, 2008.
11. Edwards, K. Vegetation History of the Southern Inner Hebrides during the Mesolithic Period. In Mithen, S (ed). Hunter-gatherer Landscape Archaeology: The Southern Hebrides Mesolithic Project 1988-1998. McDonald Institute, Cambridge, 2000.
12. Mithen, S, Finlayson, B, Finlay, N & Lake, M. Excavations at Bolsay Farm, a Mesolithic Settlement on Islay. Cambridge Archaeological Journal, no. 2, 1992. 
13. Edwards. KJ & Berridge, JMA. The Late-Quaternary vegetational history of Loch a'Bhogaidh, Rinns of Islay SSSI, Scotland. New Phytologist, no.128, 1994.

ACKNOWLEDGEMENTS

• To William for interesting local details, and to David and Morven for making it all possible.

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