Beneath Our Feet  by Denis Rattenbury

A simplified geological history of Ayrshire and region. 

 

 

A Big Crunch:
510 to 439 million years ago (Ordovician Period)
Scotland, 20 degrees south of the equator and separate from England but destined to come together.The rocks that now form the base of Scotland were on the edge of a continent (Laurentia) south of the Equator. To the south lay an open sea (the Iapetus Ocean), which was gradually getting narrower as another landmass, carrying the rocks that now form England, moved closer. It took many millions of years for the landmasses to meet. Today the “join” (the ‘Iapetus Suture’) is thought by geologists to be several hundred feet below the ground roughly in the same area as the present border between Scotland and England, an amusing coincidence. The sediments and rocks on the floor of the old ocean were being crumpled up into a huge chain of mountains similar to the Himalayas of today (the Caledonides). These great earth movements also created the Highland Boundary Fault and the Southern Uplands Fault. The crust between the parallel faults began to sink, forming a rift valley, and the Midland Centre moves down creating a valleyValley of Scotland began to develop. The crust under the Iapetus Ocean was drawn deeply down into the mantle and almost completely disappeared. Bits of it are still to be found amongst surface rocks, just inside the Midland Valley, along the coast between Girvan and Ballantrae, in an area known to geologists throughout the world. The rocks here, where we drive along “Kennedy’s Pass” and over “Bennane Head” on the A77, are known are the “Ballantrae Complex”. The geology is, indeed, very complicated and has been carefully studied for over a hundred years by experts. On the coast at Ballantrae can be seen great pillow-lavas. These were formed when submarine volcanic activity poured sheets of lava over the floor of the ancient ocean.

Lakes of Lava:
439 to 408 million years ago (Silurian Period)
The earth forces creating the Midland valley allowed a basin to form with thinning of the earth’s crust. This together with cracks in the crust allowed molten rock (magma) to push up from below into surface rocks. Volcanic lava intermittently poured out over the surface. Magma of a runny consistency could squeeze between the bedding planes of rocks to form ‘sills’. Sills may be horizontal, but can also be tilted to any angle. Magma forced up vertical or steep cracks forms ‘dykes’. In Ayrshire today we can see some remaining evidence of this. A broad upland of lavas separates the Ayrshire basin and the main central lowland belt. One 1,000 feet thick layer of lava which is more resistant than the surrounding rock now stands out as the Carrick Hills. Many old dykes and sills now stand out as landscape features all over Ayrshire and the Midland Valley, but much more volcanic activity was to affect the area later, as we shall see.

Lifeless desert:
408 to 362 million years ago (Devonian Period)
At this time there were few plants to be found on land, just lichens, ferns, reeds and mosses. The climate was dry with intermittent torrential downpours and the young mountains, devoid of vegetation, were worn away into boulders, scree and sand. A 10 mile thickness of rocks was broken up and redistributed into the Midland Valley and lakes and seas in the area. Lower rock was built up with boulders and chunks of rock carried away from the mountains by fast-flowing torrents, this became conglomerate rock. Younger (higher) rock was made up of sand and small pebbles carried by rivers that had became slower, flowing from less rugged areas as the mountains were worn away. This became smoother sandstone. Britain’s position then was 20 degrees south of the equator, in desert conditions. Iron oxide coloured the deposits red, later to form the rocks we now know as Old Red Sandstone. The land was almost bereft of life as this was the momentous time when plant life was just beginning to leave the seas to evolve into land life forms. The local seas and lakes were full of life, and we can learn about these species from fossilised fish found in Old Red Sandstone today. Pebbles of volcanic rock, remains of material spewed out by the old volcanoes, are found in the Old Red Sandstone.

Steamy swamp:
362 to 290 million years ago (Carboniferous Period)
The land mass (now Scotland and England combined) had moved north and was on the equator. Central Scotland contained a shallow tropical sea. The great mountain chain (the Caledonides) had been worn down almost completely and was now almost smooth, just very gently rolling. The rivers from this relatively flat land could carry little sediment, so the sea was clear as well as warm. This was the ideal environment for corals to flourish. The coral had 35 million years to grow in great thick layers, and it later became limestone rock. The process was interrupted, however, when more earth movements pushed up more mountains to the north. 325 million years ago the rivers running into the coral seas once again started carrying rock debris which was spread as sediment over the coral. The silt built up and later formed layers of sandstone rock. The sediment eventually choked off the rivers themselves and in these hot, humid equatorial conditions dense forests took over the stagnant swamps and marshes around sea level. Dragonflies with wingspans of 3 feet fluttered amongst the trees. Huge millipedes and cockroaches scuttled among the vegetation and debris on the forest floor. Scorpions 2 feet long hunted for prey, but king of the forest was an animal like a giant newt, 7 feet long, that was able to live equally happily in the swampy water or on land. The fossilised remains of one of these forests can be seen at Fossil Grove in Victoria Park, Glasgow. This lush vegetation would die and lie in stagnant water to form peat. After millions of years under other layers of rock the peat compressed to a tenth of its thickness and turned into coal. The result has been that limestone from the coral seas, coal from the tropical forests and shale from huge sluggish rivers containing ironstone nodules, are all found close by each other. These rocks provided Scotland with the resources it needed at the start of the industrial revolution. Because the process repeated itself many times bands of limestone, sandstone and coal were formed on top of each other. These vital economic resources for Ayrshire would be ready for industrial exploitation 300 million years later.

Fireworks:
340 to 250 million years ago (Carboniferous into Permian Periods)
At this time, as in many rift valleys, volcanoes formed around the Midland Valley. Lavas erupted and lay 3,000 feet thick over much of the Central Lowlands of Scotland. This lava now forms the horseshoe-shaped outcrop around Glasgow, the Campsie Fells, Renfrewshire and Kilpatrick Hills, and the Dunlop - Eaglesham - Darvel Hills. Many of the landscape features between Ayr and Edinburgh, including Arthur’s Seat (an old volcanic plug, with a sill forming Salisbury Crags) and Calton Hill in the city, plus Stirling Castle Rock, result from this volcanic activity. Where solidified lava is tougher than the surrounding rock it stands out today as a geological feature. The Heads of Ayr is one vent of a volcano which has been partly washed away by the sea. Loudon Hill is a volcanic plug, all that remains of a volcano, one of a line of twenty that erupted here at this time. The harbour at Troon was formed by volcanic rock pushing to the surface to form a dyke and then resisting erosion 300 million years later.

Desert again:
290 to 245 million years ago (Permian Period)
While all this was going on the rocks forming Britain were continuing to move northwards. By 290 million years ago the position was around 20 degrees north, at the latitude of the great deserts, and because of another continental collision Britain was close to the centre of a really huge continent (Pangea). For millions of years Ayrshire was desert. The wind blew massive sand dunes for hundreds of miles, just as in the Sahara today. Huge thicknesses of sand built up which have become Permian Red Sandstone rock. We know this brick-red and orange-red stuff quite well, as many of our houses in the west of Scotland are built of it. Old photographs of the quarry walls at Ballochmyle near Mauchline, where the sandstone was being extracted for building material, clearly show the lines of fossilised sand dunes. Hardly any fossils of plants or animals can be found in this rock. An exception was found recently, the skull cavity of a 2 to 3 feet long reptile that was able to live, in herds, in the extreme desert conditions.

 Death and Dinosaurs:
245 to 65 million years ago (Age of the Dinosaurs)
245 million years ago there was a major ‘mass extinction’. 95 per cent of all known species of life on earth became extinct. There has been much speculation as to the cause of this event. One theory is that the salt content of the sea increased so much that marine life died out. (Much of Britain was covered in salt at this time, the same salt that was mined for hundreds of years in Cheshire). By now Britain’s position was at about the same latitude as the Mediterranean is today. New life began to repopulate the land, dominated by the dinosaurs. Fossils of huge marine reptiles are found in various parts of the north of Scotland, but most evidence for dinosaurs in Britain is found in the Jurassic limestone areas of England. Even these fossils disappear 35 million years before the meteor is thought to have hit the Gulf of Mexico, 65 million years ago, causing the next mass extinction. 75 per cent of species became extinct, including, most famously, the dinosaurs.
 
A Really Big Fireworks Display:
65 to 52 million years ago (Early Tertiary Period)
Scotland was at 45 degrees north. The climate was much warmer and wetter than it is today. The new supercontinent (Pangea), of which Britain was a tiny part, began to break up. The Atlantic Ocean was being born! Molten rock below the earth’s crust started to well up and separate, pulling apart the two sections of continent floating on top of it. North America started to tear away from Europe. Lava forced its way up through new cracks and fissures in the crust to form a line of volcanic events running along the west side of Britain. This new rock formed the spreading ocean floor. These events produced some interesting features, such as the Giant’s Causeway in Northern Ireland, Fingal’s Cave on Staffa, and the magnificent Cuillin Mountains on Skye.

Here in Ayrshire there were plenty of spectacular volcanic events on our own doorstep. 60 million years ago a small volcano burst into life in South Ayrshire. It was one of the smallest fireworks in the pack, but we are left with the deeply eroded root of a volcanic plug, just off the coast at Girvan. This is, of course, Ailsa Craig, made of fine-grain granite (good for making curling stones) with unusual added minerals that give the rock a bluish colour. Arran had a series of firework displays all of its own. In the north, huge outpourings of lava produced the great granite blocks that have been eroded down to the mountains and hills there. King of the ‘Northern Granite’ is Goat Fell, pushing its summit up above the Old Red Sandstone around it. A few miles south, the ‘Central Complex’ formed a little later. Here a major volcano blew up and then at least four new volcanic cones grew amidst the remains. This granite mass now forms the much lower hills to the south of the B880 Brodick to Blackwaterfoot road (the road goes over ‘The String’, the narrow gap between the two highland areas). The Highland Boundary Fault runs just to the south of the ‘Northern Granite’, supporting the common argument that Arran is “Scotland in miniature”. Holy Island is the remnant of a volcanic sill. All over Arran there are places where cracks in the crust allowed lava to push up towards the surface in dykes.
 
Hundreds of dykes and sills from this volcanic activity are to be found on the mainland as well. Perhaps the easiest to see are along the shoreline. The shallow rocks extending out from the beach at Prestwick are a sill. As we drive south along the Ayrshire coast we see many lines of resistant rock jutting out into the sea in south-east to north-west lines, many of which are dykes.
 
A Big Sweep:
52 to 2 million years ago (Late Tertiary Period)
Another effect of this pressure of lava from below was to push the earth’s crust upwards along the west of Scotland. During the next 50 million years powerful forces of erosion removed much of the surface layer of young rocks from Ayrshire. It seems that any sediments or deposits were washed away quickly by powerful river systems. However, a few curiosities remain to help geologists work out what was happening here, for example, jumbled chunks of relatively young Jurassic rocks and chalk are found in the ‘Central Complex’ of Arran, unusually protected from erosion by older rocks there.
 
 
Ice over a mile thick:
2 million years ago to the present day (Quaternary Period)
Scotland continued to drift northwards to arrive at its present position. By this time the climate had entered repeated cycles of warm and cool periods, each lasting about 100,000 years. These cycles were becoming more pronounced. There have been about 20 cycles in the last 2 million years, putting Scotland into its Ice Ages. A cold spell meant that snow falling on the Grampian Highlands and the Southern Uplands in winter could not melt before the snows came the following winter. Snow compacted under its own weight and huge thicknesses of ice built up. Much ice moved eastwards into the North Sea. On the west side of Scotland, Ayrshire was in the front line, because ice from both north and south converged here before heading in a south-westerly direction. The special nature of the rock in Ailsa Craig has been useful in tracing these ice movements, Boulders of the Craig have been found in parts of England and Wales, and at Cork harbour in south-west Ireland, having been carried there by the ice. The entire area including the Firth of Clyde was buried beneath ice estimated to have been about 5,000 feet thick here when at its maximum extent 22,000 to 18,000 years ago. 14,000 years ago there was a rapid warming and most of the ice disappeared, but as always there was a complication or two. 11,000 years ago it got colder again, just for a thousand years (the Loch Lomond re-advance). One possible explanation for this is very relevant to us today. Huge amounts of freshwater from melting ice may have lain over salty water and blocked off the Gulf Stream. Since we rely on that ocean current to keep us from freezing up each winter there are fears that the global warming going on today could once again switch off the current, tipping us here in Scotland into another Ice Age!

  

“Going down”:
20,000 years ago
These glaciers and ice sheets, and their melting, fundamentally changed the landscape we can see, and what lies beneath our feet. (It also affected what lies under the water, for example deep rock basins were cut into the sea bed around Arran). The enormous weight of ice pushed the crust of Scotland downwards. The sea level also fell as huge amounts of water became trapped in ice. Both land and sea level fell, but the land dropped a little farther, so the waves pounded at a higher level along the coast during the periods of Ice Age. The result has been a series of ‘raised beaches’ which can be seen all along the Firth of Clyde and Ayrshire coast. You can see, for example at about 25 feet above present sea level, where the sea cut out beaches and cliffs that now stand some way inland and higher than the modern shoreline. Particularly broad sections of ‘raised beach’ can be seen from the A77, for example, for about a mile, as it runs southwards into Lendalfoot, cattle graze on flat ground which was carved out by waves pounding the shore. In these fields stand piles of rocks, looking a little out of place, these were once sea stacks with their feet standing in the water. A similar large tract of raised beach can be seen after you come down from Bennane Head towards Ballantrae. Around the rocky shores of Culzean Castle you can see notches cut into the cliffs and headlands indicating where the sea level was for a time. At Wemyss Bay (weem is Pictish for cave) caves exist well above present sea level.

“Going up”:
10,000 years ago to the present
Once the weight of Ice Age ice was lifted the landmass of Scotland was able to rise back slowly out of the molten mantle below. This process is slowing now, but the land is still rising at the rate of about half a millimetre a year. This uplift gave the rivers here extra energy and cutting power. The effect of this can be seen, for example, along the River Ayr, which has been able to cut more deeply into its bed to form a gorge. Good places to see the Ayr Gorge are on short walks from Auchincruive, Failford, or at the Ballochmyle Gorge.
As the glaciers moved from highland areas across Ayrshire billions of tons of rock debris was picked up and then dumped across the lowlands. Much of the soils of Ayrshire are predominantly glacially deposited sediment. Some of this contains marine shells, indicating that some ice must have moved from the sea onto the land. Just before the ice melted some of this was pushed by the remaining ice into low mounds called ‘drumlins’ (drumlin is Irish for smooth hill). These hills are to be found all over Ayrshire, but they often occur in groups. There are swarms of them around Glasgow on both sides of the Clyde. Sauchiehall Street runs down between two drumlins. To the east of New Galloway there is a drumlin field. Throughout Ayrshire we are well used to seeing low, well rounded hills in our landscape. Some of these are drumlins and others are resistant outcrops of volcanic rocks which have been smoothed down and modified by ice.
 
Soon after the retreat of the ice the surface of Ayrshire was covered by birch forest. The temperature continued to rise, and oak, ash and elm trees took over. These forests helped to make the material deposited by the ice more fertile, by adding humus. Small family groups of Mesolithic hunter-gatherers moved along the shoreline. Their shoreline has become what are now our raised beaches. They left evidence of their former presence in the form of stone tools, shell-mounds, middens and camp fires found, for example, at Ballantrae where the River Stinchar flows into the sea. Man had arrived on the scene.
 
A relatively recent natural feature of the landscape along some parts of the coast is blown sand. This is a feature along the stretch between Saltcoats and Prestwick. A very positive result has been the formation of the ‘links’, and the great sport of golf all along the coast through Troon and down to Turnberry. On the negative side the sand has caused its fair share of problems. The town of Ayr had a major battle with gale-blown sand, a problem which became serious at the end of the 14th century. The Sandgate was regularly buried in sand. In 1380 land was offered to anyone who was prepared to try and reclaim it from the dunes, but no one seems to have taken up the offer.
 
Today:
Further inland felling and burning of the forest began thousands of years ago but accelerated in the industrial age. Land was cleared to create farmland. Timber was wanted for housing, shipbuilding, fuel, and to make charcoal for use in iron smelting and lime burning. As coal mining and heavy industry developed in Ayrshire, man’s influence on the landscape increased. And yet, even this industrial landscape has, to a great extent, already disappeared. New forests are being planted and farms improve the soils provided by volcanic and glacial action. Farmers are now even being told to reduce certain types of production. Giant earth-moving machines allow open-cast coal mines to remove whole sections of hillside, but eventually even these scars can be returned to a landscape that looks surprisingly natural. Two years ago the landscape along the Ayr to Glasgow road was disrupted by an army of machines. Hills were cut, removed and repositioned. Rivers and streams were being rerouted. Vast tracts of country turned into a brown, ugly quagmire. Yet, already, we race along the new M77 and enjoy the green scenery as well as the speedy transport. Man seems to be ruler of nature. But it does us good to think back millions of years to see how puny we really are.


 
 
 

 

 

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