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 flowed into a lake, taking with it sand and silt out of hills even more unbelievably old. Sands and silts were washed down easily, with no roots to hold a soil together, no soil. There was no life on land. In hot dry seasons the winds blew dust into streams feeding the river. In wet seasons rain washed sand 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, apparently for a very long time, around 200 million years. 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 is found in the sandstone but compressed sand. Almost nothing. In a very few places on Earth, traces have been found of what might have once been life on Earth alive over 700 million years ago. One of those places is Cailleach Head on Scoraig, 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, about 1,000 million years old. Possibly, towards the end of the 200 million year life of the lake, a little green slime in surface shallows, life, was washed over by sand and entombed until the 20th century.
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. Possibly not very much.
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 here shows where our sandstone disappears underground, on the leading edge of the zone of collision where Avalonia and Laurentia collided over 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 Avalonia, now covered with coal, chalk, limestone, sandstones, mudstones and volcanic rocks, all created in the last 500 million years.
The purple here is the Torridon sandstone of the ancient lake sandflats laid down over 1,000 million years ago.
The orange scattered in with it is even older ~ Lewissian gneiss, formed in volcanic action 1,700 million to 2,700 million years ago. It was created half the Earth's lifetime ago, and In most places in our region the gneiss is deep underground below the sandstone.
After two juggernaut trucks collide head-on, the mangled conglomeration of their wreckage might still creak, snap, and shift, hours after the smash. Long after the collision of two continents, the land groaned, tore, and shifted.
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.
Snowfall in December 2010 picks out the Coigach fault with unusual clarity.
The darker more knobbly land, locally known as Carn Dearg, is gneiss.
The nearer white land is sandstone covered with peat and heather moorland.
The lochan just in view in the dip between them sits inside the line of the fault itself.
Fast forward from the dawn of the mammal age and the birth of the first monkeys around 50 million years ago past the coming of Earth's first grasses around 30 million years ago past a funny little primate walking on two legs in Africa around 7 milion years ago, to just 100,000 years ago. Lions roam where London will stand. Some of their bones will be 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 100,000 years ago, humans 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 more...
In time the snow compressed into ice sheets, and began to slide down mountainsides ... across the north more ice and snow reflected more sunlight back into space, cooling Earth more, until snow fell here even in summer. The last Ice Age was the deepest in millions of years. In time, ice over 1,000 METRES DEEP glittered where Scoraig is now. It stretched hundreds of kilometres away south. The Ice moved, and all traces of the land below were stripped away to naked rock.
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 an Ice sheet. The pattern of the grooves shows that at different times the Ice moved in different directions. Sometimes it moved west towards the sea out of rivers of Ice from mountains inland far higher than they are now. Sometimes it moved south in a general ooze of Ice from the north pole across Asia, America and Europe. The Ice over Scoraig in time reached a kilometre thick, and stayed that way thousands of years.
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