Geology of Scoraig

Written by Steve Webster, a crofter on Scoraig, September 2009

The British isles contain more geological diversity than any other land area of our size on the surface of Earth.

From Scoraig we can see the biggest single reason why.

Geologically speaking, Scoraig is not in the Highlands of Scotland.

Scoraig is beyond the Highlands. We can see the western edge of the Highlands, just beyond the snow covered mountain of Beinn Ghoblach, the nearest in this view.

The tip of the Scoraig peninsula, Cailleach Head, has been called one of the finest cyclothermic sites in Europe, and is a Geological Conservation Review area.

So what is a cyclotherm, and what is special about Scoraig ?

The story begins somewhere on Earth 1,200 million years ago. The most advanced life forms on the planet are algae in water. Today, 10% of Earth's land surface is limestone, made from crushed beds of old seashells and corals. There was no limestone 1,200 million years ago. No life existed capable of making a shell. A river flows into a lake. It brings with it sand and silt from higher ground, out of hills even more unbelievably old. Sands and silts are transported rapidly, as there are no roots to hold soil together. There is no soil. No life on land. In hot dry seasons the wind lifts dust, blows it into streams feeding the river. In wet seasons rain washes sand into streams, into the river, down into the lake.

The flat bedded tabular sandstones were deposited from fast-flowing shallow flood water crossing sandflats bordering the lake. When the lake level rose and temporarily covered the sandflats, the shallow water was forced to decelerate and deposit sediment as straight-crested sand waves. Some of the sand was carried beyond the sandflats and across the lake floor, thus accounting for the ripple-laminated sandstone bands.

From The later Proterozoic Torridonian rocks of Scotland by A. D. Stewart

It was a big lake fed by a big river, and it went on a very long time. The Torridonian sandstones are 5,000 metres thick. The river may have contained dissolved mineral salts. If so, the lake had an outlet river that carried the salt away before a dry season could cause any salts to be dropped out of the water on the sand bed of the lake. Almost nothing has been found in the sandstone but compressed sand. Almost, but not quite entirely. In a very few places on Earth, traces have been found of what might have once been life on Earth that lived over 700 million years ago, and one of those places is Cailleach Head, where in 1907 two geologists discovered a scrap of organic remnant of what might have been life. The youngest sandstones made from the sandflats of the ancient lake bed anywhere on the land surface today are at Cailleach Head, the western tip of Scoraig. They are about 1,000 million years old. It appears the river fed the lake for 200 million years. Possibly, towards the end of the life of the 200 million year old lake and river system, a little green slime in the surface shallows of the lake was washed over by sand and entombed until 1907.

On the cliff face of Cailleach Head are its cyclotherms ~ cycle after cycle of sandstones and silts spread over the lake bed in different seasons, down and down into the sea, and 500 metres further down below the sea bed. What happened on the old sandflats in the next 300 million years is unknown.

Earth between about 700 - 600 million years ago ~ the mother of all Ice Ages, when according to current Earth science theory the sea froze all over Earth, all the way to the equator.

What life survives is under or in the Ice.

Somewhere down there the old sandflats are undergoing compression.

Around 600 million years ago came meltdown.

About 100 million years later. In the sea, animal life is growing, beginning to make shells, legs, mouths... The first eyes are open. Lichens and mosses are spreading inland, and life is moving up the rivers.

The red circle on the map shows the location of the Scoraig sandstone, already 500 million years old, on an island continent near the equator.

The yellow circle shows another bit of Earth crust which will become England, Wales & southern Ireland.

Another 56 million years later. Sharks swim in the ocean, along with the little fish who is our ancestor.

The red circle has moved south, on an island continent called by geologists Laurentia. The yellow circle is on another island continent, Avalonia, that has torn off from the southern supercontinent which contains most of Earth's land.

Between the two is another island continent, Baltica. The three islands are on a collision course.

Another 33 million years have passed.

The first insects are moving on land ~ spiders, centipedes, mites, all without wings. The king of the beasts is Pterygotina, a kind of scorpion the size of a man who lurks in the shallows. Fortunately there are no men. The tallest living thing on land, Prototaxites, a fungus, grows 8 metres tall.

The three island continents are colliding.

35 million years later.

The first small woody trees are growing. There is no vegetation on land taller than 1 metre.

The red & yellow circles have merged into one purple circle containing the future British isles. It is south of the equator. Avalonia, Laurentia and Baltica have crunched into one island continent. In a disused quarry in Poland in 2009, a footprint was found, estimated to have been made 397 million years ago, of something that was not an insect. It is the oldest such footprint so far found on Earth.

34 million years later.

On land are giant tree ferns and giant insects, including a dragonfly with a 75 cm wingspan. Giant plants die, fall in swampy ground, and are buried under more before they can rot. They make massive beds of peat which will in time be buried under more sand and silt, or under coral when sea covers them. They will make the seams of coal we burn.

The little fish has crawled out of the sea onto land, the ancestor of all 4 legged animals, all reptiles, mammals, birds, and us. What will become Scoraig is crossing the equator.

Through the purple circle runs a mountain range the size of the Himalaya today. The Highlands of Scotland today are the last worn-down stumps of that mountain range.

100 million years later.

The first reptiles have appeared, with a dry skin so they can travel further from water. They are spreading inland.

Earth's land masses have helpfully crunched together to help them get around.

60 million years later.

Flowers are spreading over land.

What will be North America and Greenland have torn away from what will be Europe and Asia. Between them the future British isles contains bits of both, including the old lake sandflats long ago compressed into sandstone.

Some reptiles have grown huge. Earth is in the dinosaur age. It will last another 130 million years.

100 million years later

Broadleaf trees are growing, and the dinosaurs are still going strong. Some swim in the sea. Some fly.

Here and there on Earth, mammals guard newborn young underground where they feed on a liquid the mother secretes from her skin, so she can keep them for the first weeks of life away from the exciting action above.

50 million years ago.

The dinosaur age is over.

The age of the mammals and birds has begun.

There is no need for a purple circle. You can find the British isles, although most of it is under a shallow sea where corals build what will become the White Cliffs of Dover and the chalk landscapes of eastern England.

The images above were downloaded from www.discoveringfossils.co.uk.

Through those hundreds of millions of years, we know very little of what went on above the old lake sandflats. Later ages have stripped away whatever covered them, apart from a few scraps. A few miles north of here, just outside Ullapool is a limestone quarry, where was once an ancient coral reef. On the isle of Skye, south of here, last winter a fragment was found of a Jurassic turtle, some 170 million years old. Skye was made in massive volcanic eruptions around 60 million years ago as north America and Euro-Asia tore apart and the Atlantic ocean widened. In places volcanic lava flowed over beds of older rock and preserved the dinosaur evidence it contained. You can visit Skye today and see a three toed footprint embedded in hard rock which was mud when a megalosaurus trod in it.

East of here, the sandstone dives underground, beneath the Highlands.

The thick black line in this image shows where the sandstone disappears underground. It is the leading edge of the zone of collision where Avalonia and Laurentia crunched together around 400 million years ago. Everything northwest of the line was once Laurentia. Everything southeast is a jumble, a pile-up we know now mostly as the Highlands of Scotland, down to another line running roughly from Berwick-on-Tweed, close by the Isle of Man and across northwestern Ireland. Everything southeast of that line was once Avalonia, covered with coal, chalk and limestone, sandstones, mudstones and volcanic aftermath of the last 500 million years.

The purple here is Torridonian sandstone, made from the ancient lake sandflats.

The orange is an even older rock, Lewissian gneiss, formed in volcanic action 1,700 million to 2,700 million years ago, half as old as Earth. In most places in our region the gneiss is deep underground below the sandstone.

When two juggernaut trucks collide head-on, the mangled wreckage might still creak, snap and shift hours after the smash. So, long after the continental collision that made the British isles, the land groaned, tore and moved.

On a map of Scotland you can see how it tore along the line of Glen Mor, Inverness to Fort William, making Loch Ness. What is now northern Scotland, northwest of the Glen, has been shoved a long way northeast. It started over 300 million years ago. In the last 150 years, since the invention of seismographs, slight shifts on the Glen Mor fault line are still occasionally detected.

Across Scoraig runs the Coigach fault, a land shift which exposed the older gneiss seen on our north coast at Carn Dearg. The fault line runs past Carn Dearg and the nearby lochan it created, across Scoraig inland of Cailleach Head, under the sea and through Stattic Point on the far shore of Little Loch Broom. It runs NNE - SSW, roughly the same way as Glen Mor. Geologists are still putting together the story of the aftermath of the collision. It is like putting together two jigsaw puzzles when the pieces are all in one box and many are missing. On Scoraig, the Coigach fault wrestled to the land surface our oldest and youngest rocks, at Carn Dearg and Cailleach Head, a short walk apart.

From the dawn of the mammal age 50 million years ago,we fast forward through the coming of Earth's first grasses around 30 million years ago and a funny little upright walking primate in Africa around 8 milion years ago to just 100,000 years ago. Lions roam where London will stand. Some of their bones were found in the construction of Trafalgar Square. Up here almost 1,000 kilometres further north live mammoth, giant elk, sabre tooth tiger, giant beaver... The mountains stand far taller than any mountain in Britain now. A few humans might have lived here then, with brains as big as ours, inside a differently shaped skull.

We will never know. The last Ice Age began about 70,000 years ago. Snow fell in winter, and high in the mountains the snow did not all melt before another winter brought more snow, and another winter more... In time the snow fields compressed into ice sheets and began to slide down mountainsides, into valleys. Across the north more ice and snow reflected more sunlight back into space, cooling Earth more. The last Ice Age was the deepest in millions of years. Less than 25,000 years ago, Ice over 1,000 metres thick glittered where Scoraig is now, and for hundreds of kilometres away south. All traces of life below were stripped away back to naked sandstone and gneiss.

The coldest depth of the last Ice Age was less than 25,000 years ago.

Under the thin peat that covers most of Scoraig is a bed of hard packed sand dotted with boulders of sandstone of various sizes and an occasional lump of gneiss. These are the grindings of the last Ice Age dumped here when the Ice melted away. The sand is hard, too hard to get a spade into it. It is returning to sandstone.

Slabs of old sandstone outcrop on the surface, as in this photo, etched with grooves where rocks were dragged along the bottom of a slow moving Ice sheet a kilometre thick.

Looking west along the southern shore of Scoraig.

A glacier here once scooped out a deep valley. The seabed is 70 metres deep under water visible in this photo.

All around the shoreline on both sides of the loch, the last few metres of land dip steeply into the sea, as seen here. After 60,000 years, the last Ice Age melted down rapidly. Sea level all around Earth rose about 130 metres. The weight of a kilometre of Ice was lifted off the land here, and in sheer relief the land rose, creating this shelf about 5 metres above current Earth sea level as of September 2009.

Cailleach Head