'Black Rocks', Crossapol  |       

Geological evolution of the Isle of Coll

Our humble Isle of Coll represents a mind bending 3 billion years of earth history! Hopefully it won't seem that long to explain!

The rocks of Coll, Tiree with much of the outer Hebridean Isles together with a narrow strip of the mainland coast between Loch Torridon and Cape Wrath, are pretty ancient. At around 3 billion years old they are amongst the oldest known to humankind. They also make up what is referred to as the 'basement' rock, on top of which successive layers of geology representing the rest of evolutionary time as we know it have been built.

To try to put the vast expanse of time elapsed into some sort of digestible context, if all of earth history were condensed into 24 hours and the earth formed at midnight: at

  • 3.45am the geological record begins, with the oldest known rocks found in Greenland. There is evidence of unicellular organisms and at this time, the earth already had a well developed crust and oceans.
  • 5.30 am the first plant-like life forms to discover photosynthesis appear and subsequently set about enriching the atmosphere with oxygen.
  • 8 am the parent version of our very own Lewisian Gneiss was born, from a combination of volcanic activity and the settling out of sediments in a shallow submarine environment.
    Half the day goes by and drags on into the evening, without much happening ...
  • 9 pm the atmosphere becomes sufficiently oxygenated to allow an evolutionary 'explosion', or a kind of biological arms race, with more complex and diverse forms of life arriving
  • 11pm-ish and lasting no more than 40 mins was the age of the dinosaurs (they left some of their footprints on Skye)
  • 11.40pm mammals appeared, exploiting the void left by the Dinosaurs
  • 11.58pm our hominid ancestors arrived, maybe dragging their knuckles
  • 12 pm midnight (in time for last orders?) here us modern humans are, and in only a tiny fraction of a second......

Now, back to the rocks.

And so the vast majority of Bedrock on Coll (it rarely goes unnoticed, there's a lot!) is made up of 'Lewisian Gneiss' in reference to the Outer Hebridean Isle of the same name where it was first described.

With its coarsely crystalline mineral structure, Lewisian Gneiss is an incredibly hard and resilient kind of rock, as anyone who's ever tried to drill through it will know.

This type of rock has undergone 'metamorphosis'. As the parent rock became subject to increasing heat and pressure within the earth's crust, it underwent some fundamental changes. Intense shearing forces caused the component minerals to align into 'foliations' and extreme heat caused them to separate out and recrystallise into the light and dark bands for which gneiss is renowned (known as 'Gneissic banding' ).

As Lewisian Gneiss is so ancient and has been extensively reworked, Geologists can only speculate as to what kind of ancient environment its parent rocks formed in. Perhaps this is why the assemblage as a whole is known as the 'Lewisian Complex'. It is thought to have taken a staggering 2 billion years to form!

Though it is believed to be mostly igneous (once molten), probably volcanic in origin, there are also sedimentary elements (metasediments) which interestingly (or not!) include a type of marble (formerly limestone) once quarried for its aesthetic appeal at Balephetrish on Tiree. Here on Coll however, only small isolated lenses of a similar type of marble can be found dotted around the west end .

The upstanding prominence of Ben Feall consists of these Lewisian metasediments. The steeply dipping layers of rock erode along planes of weakness where the mineral layers are less resistant. The result is the jagged nature of the sea cliffs here and they are easily (and safely!) viewed from Feall beach. This hard, pale quartzite/psammite originated mostly as sandstone deposited in an ancient shallow sea.

The Lewisian complex has undergone several recognisable traumatic events throughout its history.

Initially, the collision of ancient tectonic plates subjected some of Coll's Lewisian Gneiss to such intense heat and pressure it began to partially melt.

Conditions extreme enough to cause this are associated with the tectonic processes of mountain building, when two continents collide. Here the earth's crust becomes significantly crumpled and thickened that the base is pushed down towards the earth's mantle, where phenomenal pressures and temperatures preside.

The resulting rock formations are 'Migmatites'. They are found at the very deepest roots of mountain ranges, and although not completely igneous they are also too far gone to be considered entirely metamorphic. The resulting rock looks very much like Gneiss but often shows extreme contortions in the banded layers known as 'Ptygmatic folding' (resembling intestines!), in addition to various other visually splendid features with equally jargonistic names

Through partial melting some of the more buoyant mineral components of the rock become mobilised. Silicates (an example being quartz) tend to be most mobile and are first to melt (forming the 'leucosome' ), while other more resilient minerals can remain in their solid state (forming the 'residuum').

Where shearing forces act to pull apart a relatively resistant band of rock 'Boudins' can form, (named after the French for a 'string of sausages'). Isolated rafts of resistant rock are known as 'Schollen' (German for 'clods'). As the melt progresses even further they degrade into 'Schlieren' (German for 'streaks') with molten material veining through these rafts. Some of these rafts may even rotate within the more ductile and fluid component of the rock. On close inspection the foliations of this 'new' rock (or 'neosome') can be seen to flow around the the more resistant, 'competent' phase. The mobile leucosome tends to migrate and accumulate around areas of lower pressure within the rock mass, forming net like patterns and veins inside structural weaknesses.

These processes are responsible for the sometimes strikingly colourful rock formations of Coll. The beautiful orangey-pink banding at the colloquially named 'Red Rocks' is due to high concentrations of feldspathic minerals, which is a principle component of Granite. Green streaking here is due to the presence of epidote , another by-product of metamorphism, though from a later less intense phase.

At 'Black Rocks' , on the south end of Crossapol beach, the rocks are concentrated in darker 'Mafic' minerals which are chemically richer in Magnesium and Iron. These tend to represent the more resilient mineral fractions.

When the fractionation of minerals is well developed plumes of molten material can part company from the rest of the rock mass to form detached plugs and sheets. The resulting rock can have amazingly granite like features and granitic veins cut through the gneiss in places.

A very course grained version of granite known as 'Pegmatite' can be seen close to the road shortly before the cattle grid at the Feall RSPB car park, in the form of a couple of well rounded rocky knolls either side of where the road chicanes slightly . There is also a large boulder about 200m along the track to Feall which may have been detached and carried there by glaciation.

Along with some of the lighter melt fractions, mineral rich water and other volatile gases can vent through fractures in the crust. Such hydrothermal activity can deposit minerals rich in valuable metals. In spite of a reference (Hamish Haswell-Smith 1996) to lead having once been mined on Coll, there seems to be no evidence of this.

Further geological trauma on Coll is present as a set of magmatic intrusions known as a 'dyke swarm', where molten rock was injected into a host rock (our Lewisian Gneiss) in multiple parallel lines at the same time. In three dimensions, the magma forms narrow, vertical sheets. However, to us, on the eroded surface it appears as an even, straight band of darker rock. These more contemporary features always cut through any earlier deformations in the existing rocks and so are useful in assessing the relative timelines of geological events.

As the molten rock comes into contact with the host rock it cools and solidifies rapidly, particularly along its margins. The more rapid the cooling the finer the crystalline structure and the dyke rock tends to erode in a blocky manner along lines of weakness formed by cooling contraction joints. This intrusive alien rock (mostly Dolerite) is associated with volcanic activity and is described as 'Basic' or 'Mafic' as its mineral makeup is rich in Magnesium and Iron.

These Dolerite dykes are numerous and generally rather obvious features particularly across the wave washed rocks of Coll's shores. In places along the coast one may encounter a deep narrow chasm where coastal erosion has carved out the less resistant dyke rock from the Lewisian Gneiss hosting it. Some of these natural clefts can be more than 10m deep and you certainly wouldn’t want to fall into one! On the plus side they form a nice microclimate for ferns and liverworts!

The first dyke swarms occurred around 2.4 billion years ago and are known as the 'Scourie' dyke swarm. They have themselves experienced alteration and deformation, indicating further tectonic activity. These very ancient dykes are not as obvious as some of the younger intrusions and appear as concordant mafic sheets within the Lewisian gneiss.

The evidence for this metamorphic episode can be found on examination of the altered mineral assemblages in the dyke rocks. These dykes have large concentrations of amphiboles, a group of minerals formed in a specific range of temperature and pressure, along with other indicator minerals such as garnet and epidote. It appears the conditions were not quite as extreme as those under which the Lewisian Gneiss was formed and the type of metamorphism is known as 'retrograde'.

There is an enormous period of time missing from the geological record on Coll after this phase of activity. The ancient uplifted land mass was slowly worn down to its roots during a long period of tectonic stability, where really not a lot happened. Though, if any strata had been deposited in the meantime, it has long since eroded away.

We can only piece together what may have happened on Coll by looking at the rocks which overlay the Lewisian gneiss basement in other areas. Where the younger layers of rock occur, they lie on the surface of an ancient rugged landscape which was exposed to the elements for many hundreds of millions of years. This is known as an 'unconformity' and often weathered pebbles and rocks of the underlying rock type are found within the next layer to be deposited. Over a time period of more than 200 million years a great depth of red sandstones built up several km thick. Found mostly in the north west Highlands the 'Torridonian supergroup' formed in a vast braided river system, on the flanks of an eroded mountain chain. It is thought these sediments were formed in the initial phase of continental rifting (such as the modern day east African rift valley).

Rum is one of the nearest localities to Coll where rocks of this age outcrop, however, they are also found on the bed of the Sea of Hebrides. The beach at Hogh has a localised abundance of red sandstone pebbles and slabby fragments which suspiciously resemble Torridonian sandstone. Although no bedrock of this kind is visible, or been recorded on Coll, they could have been washed ashore from a submarine outcrop or even possibly been dumped by glaciers.

After this, at around 500 million years ago, the rifted landmass opened up into an ocean (known as the Iapetus) as a new succession of shallow marine sediments were laid down. Though absent from Coll these rocks can be seen on Skye and the north west mainland coast, along with even younger sequences.

This ocean eventually began to narrow again resulting in the collision of what is currently north west Scotland, Greenland and north America, with Scandinavia, and the rest of Britain and Europe. 490 – 390 million years ago, a new mountain range was born and this episode of mountain building takes its name from The Latin for Scotland, 'Caledonian'. The Infamous Great Glen fault symbolises one of the sutures of this complicated 3 phase collision, with the geology to the north west of it more akin to that of north America. The physical join between Scotland and England/Europe runs roughly parallel with the geographical border!

Another 340 million years passed by before the next phase of geological activity was embellished into the rocks of Coll! In geological terms this was only very recent. Another dyke swarm happened around 60 million years ago, about the time of the Dinosaurs. These appear to coincide with the birth of the Ocean we now know as the Atlantic.

As tectonic plates pulled apart and the earth's crust stretched and thinned (yet again!), a series of volcanos formed throughout the north west of Scotland and northern Ireland. Their activity is responsible for some of the most well known landscape features of Britain such as the Giant's Causeway (Antrim) and Fingal's Cave (Staffa). Our nearest active volcano was on Mull where only its deeply eroded roots remain. Other centres of eruption were on Arran, Ardnamurchan, Rum, Skye and St. Kilda along with a submerged centre south and west of Tiree (Blackstone rocks).

So much more is known about this tumultuous time compared to the origins of the Lewisian complex as the associated rock formations have been the focus of numerous studies, and are of course much less deformed! It is thought that the resulting Basaltic lava flows erupted periodically over about a million years and were up to nearly 2km thick in places! The nature of the eruptions was not unlike those of present day Iceland, where the Atlantic ocean continues to rift apart.

Unfortunately for us, we have none of the truly exciting stuff, such as the fossilised remains of a tree caught in a lava flow (MacCulloch's tree, Mull) or the roots of an extinct volcano (also Mull, maybe the next place to visit?). All we have is yet another swarm of basic dykes, which appears to have emanated from the Mull volcanic complex.

This could be due to the fact that Coll was located on the edge of what was an area of very localised volcanism (known as the 'Hebridean Igneous province'). Also Coll has been displaced from it by a major fault, not currently known to be active. The Camasunary-Skerryvore fault is one of a series of 'half graben' faults trending NE-SW across the Hebrides. It probably appeared at the same time as the Great Glen fault and follows a similar trend in the north as far as Skye and extending to Tiree in the south.

Previous movement along this fault, which runs close to the east coast of Coll, has elevated the land mass on our side, with relative downward slippage on the Mull side. This is pretty much the reason why Coll and Tiree exist at all! The layered sequence of lavas which form the Treshnish Isles with their distinctive profile, and the stepped cliffs of Mull, extend below the sea as far as Coll where they are truncated by the fault.

Another series of possibly older faults, mostly at the east end of Coll with a NW-SE orientation, cut through the Lewisian gneiss (don't worry they are also no longer active!). They have influenced Coll's topography as historical movement caused localised weakening in the rocks. The general east - west orientation of many of Colls east end lochs have formed along these lines of weakness. As does the south edge of Bousd harbour, and the inlet here is almost certainly a product of differential erosion along this line.

Even the sheltered bay at Arinagour (Loch Eatharna) runs adjacent to one of these fractures. As it cuts through the Island, almost south to north, it bifurcates into a Y where the burn meets the estuary. The road follows one limb of the fault to Arnabost with the other running along the watercourse towards Gallanach.

Over the last 2.5 million years the topography of Coll's ancient landscape has also been altered by intermittent glaciation, with the most recent Ice Age being at its maximum around 20 000 years ago. During this time Coll, along with the rest of Scotland, would have been covered by a vast ice sheet extending westwards to the continental shelf edge. The ice flowed from the higher ground of the mainland and diverged around the highest point of Mull, which remained an ice free 'nunatak'.

Once the ice retreated, somewhat counter intuitively, the sea level actually fell! This was purely due to the great volume of ice being lifted, which over time, allowed the landmass once below it to gradually rise. This 'isostatic readjustment' is the reason why 'raised beaches' can be found all across the Scottish coast, including Coll. This is illustrated in the distinctive profile of the Treshnish Isles especially the Dutchman's cap where the 'cap' sits on a level platform which was once submerged below sea level.

The action of ice on the resiliently hard Lewisian gneiss resulted in the characteristic 'cnoc-an-lochan' type landscape we see today particularly at the east end of Coll, with rounded rocky knolls and boggy ice scoured hollows.

Where the underlying geology of the Lewisian Complex has more metasediments, the effects of erosion have been enhanced. In the west end of Coll, the low lying ground which cuts a swathe through the island from Hogh beach to Breachacha, is underlain by this slightly less resistant rock type. However, as it turns out this is a handy place to put an airstrip!

The effects of glaciation are widespread across Coll. Striation lines gouged by rocks trapped in the ice can be seen on bedrock surfaces, following the direction of ice flow. Glacial erratics carried from distant places have been dumped such as the one lodged high up on Ben Hogh .

Any superficial deposits overlying the bedrock are referred to by geologists as 'drift' and have been deposited since the last ice age as glacial till for example.

Once the climate warmed, about 11 thousand years ago, the first soils would have began to form. Overlying mostly Lewisian gneiss, they were probably shallow and stony with poor drainage and an acid pH. Initially they were colonised by woodland scrub, until early settlers cleared much of it. Several thousand years ago when rainfall levels increased, the conditions for peat formation were ripe.

Inland, peat bogs accumulated great thickness of partially decayed plant matter in the poorly drained hollows left by the glaciers. Close to the exposed shore, windblown sand built up into great dune systems, with some of the highest in Scotland found on Coll . Where it contained a high proportion of calcium rich shell fragments, this windblown shell sand served to improve the land inshore. With the help of generations of crofters who applied seaweed to their crops, it became a fertile floral grassland . Thus the machair was born, providing us with the most bio diverse and beautiful of natural habitats, and a reason why so many of us enjoy Coll. Globally unique only to the west of Scotland and parts of western Ireland, it is thankfully still home to some of the rarest plants and animals in Britain.

In amongst the dunes and machair of Coll, you may come across the fossilised roots of an earlier generation of dunes standing proud . The course fragments of shell sand have become cemented together by calcium rich groundwater percolating through the voids. Given the right conditions, the dissolved calcium carbonate has recrystallised, hardening and preserving what remains of the dune. The wind generated 'cross bedding' structure is preserved in cross section in places , though the formations are often crumbly and friable, as they break down readily on exposure to the elements.

Described as an 'indurated aeolian Calcarenite' (MacTaggart 1996) it is a form of 'calcrete'. Not really a true rock as it hasn't undergone the full range of physical and chemical changes involved in the 'lithification' process, or conversion to rock.

Sheep find them useful scratching stations!

Oct 2020. © Melinda Cottrell, BSc (Hons) Geology/Biology (Major Geology), PgDip (Countryside Management)