Caledonian mountain prelude: Volcanoes, rivers and big boulders in Scotland

Four hundred and twenty-something million years ago, a river flowed through the landscape that would one day become Scotland.

The river flowed across a plain, green but with only low plants and brushes. In this time, the late Silurian, plants were on the verge of evolving stems and making forests, but not yet, their tiny stems reaching no longer than a child’s foot towards the sky.

The river carried mainly sand. It left the sand in the outer swing where the river channels turned and twisted, and as the river moved back and forth, the sands overlapped into broader sheets. Now and then, when the river burst through its banks during flood, those bursts created fan sheets of sand.

Then, after some millions of years, the river became wilder. It thundered along and carried large boulders, tumbling and bouncing them until they became rounded, polished like big eggs. It was as if some big giant had lifted the land up in the east, making it steeper, giving the water more energy.

The almost-Caledonian conglomerate at Crawton, with meter-sized boulders. On the upper image, a coarse conglomerate drapes over a finer one – and is again capped by a lava flow.

The land had become steeper, and the giant lifting it was, of course, plate tectonics. Continents had started to clash. From the east came Baltica, a continent roughly comprising the present Scandinavia, Baltics and east to the Urals. From the west came Laurentia, roughly North America and Greenland – and northern Scotland. In between was Avalonia, a small continent – or large island, comprising the rest of the British Isles. In splendid isolation, then as now, until it got squeezed between the biggies. This collision pushed land high up in what is now the North Sea, and it was these heights that eroded to become the boulders.

The collision continued on for ten more million years or so, into the early Devonian time. It made one of the tallest mountain chains the world has seen, possibly even out-talling the Himalayas. We still find the old roots of this mountain chain as the mountains in Norway, and the Scottish highland, which also gave them name: The Caledonides, after Caledonia, the old Roman name for Scotland.

Caledonides EN

A simplified map of the Caldonian mountain belts, in orange, around 420 million years ago. The continents Baltica and Laurentia collided, squeezing the north tip of Avalonia in between. Much later, the Atlantic ocean opened roughly along the same mountain belt. (Woudloper, CC BY-SA 1.0, via Wikimedia Commons).

But that was later. In Scotland-to-be, anno minus 420-ish million, the rivers piled up sandstones and boulders to a thickness of possibly 9 – yes, nine – kilometers in the area south of Aberdeen. The pile then thins westwards. The reason it could reach such a thickness is that the colliding continents not only pushed land up, but also down in front of the mountains. This foreland bulge was basically a mega-version of the bulges that appear if you take a foam mattress and push it in both ends – and as it deepened, it also made space for the sediments to pile into.

And then, there is more. Volcanoes. Volcanoes always add that extra spice to geology. The spice probably origins from the closing of the ocean that was between the continents. When the ocean closed, the heavy sea bottom sank in beneath the lighter continents, as it does in the “Ring of Fire” around the Pacific. Remnants of that slab probably melted, partly, in the top of the mantle, and came up as volcanoes.

The conglomerate and lavas upon them at Crawton. The contact is rugged, but quite flat, suggesting that there was a break between the conglomerate and the lava. Note the holes in the conglomerate, left by once-removed big boulders, and the characteristic radiating six-sided basalt columns in the upper image.

Of course, I did not find out all that myself just by looking at some Scottish outcrops. As large and complex geology stories often do, the story of the Caledonides is the result of many geologists looking at rocks, hammering at rocks and lab-working with rocks. The story of the rivers with boulders and sand comes largely from the works of professors Adrian Hartley, John Howell and their accomplices at the University of Aberdeen.

Hartley and Howell were guides on a field trip to these rocks in September 2023 – a little break in a meeting of exploration managers in Norwegian oil companies. Finding oil is my day time job, and the exploration managers meet regularly to exchange ideas and experiences.

And sometimes we go looking at rocks, because deep inside even the most well-suited, C-suited exploration manager, there is a geologist that longs back to field days with dirty boots and worn fleece, hammer and compass in hand.

A waterfall plunges from conglomerate cliffs at Crawton, a small bay next to the main locality. Geologists for scale in the lower image.

The images in this post were taken at two places, just south of Aberdeen. The first is the cosy town of Stonehaven. We walked along the beach, looking at the broad sheets of the older river sands, with some clay in between. It was like walking through the pages of a very big book, or an exhibition of strange, beautiful patterns of art. The rocks are turned on the side.

River sediments at Stonehaven: Mostly sand, but in between are shale from calm river plains, and gravel from fast streams in outer wings. In the upper image, the soft shale is eroded and and has given way to the sea water at the shore.

After deposition, when the Caledonides and all that jazz was over, the sands were bent up by movement along the Highland Boundary Fault.

The Highland Boundary Fault is a large fault, that crosses the whole of Scotland, from Loch Lomond in the west, to Stonehaven in the east. On its north side are the Highlands, on the south side is the Midland Valley, with Edinburgh. Around 360 million years ago, on the border in time from the Devonian to the Carboniferous, compression along the Highland Boundary fault pushed the sandstones up, bending the closest sandstones into the steep position.

Artwork by the river sands at Stonehaven: Sand flows cutting into each other creates wave patterns back and forth, accentuated by thin layers of clay. The sandstones were later bent and faulted, and tilted into steep position. The strange web-like patterns are called tafoni, and come from the sea salt etching on the surface.

The second place we visited was Crawton, a bit further south and upward in the sediment pile, to the conglomerates.

The conglomerates look like some Silurian giant has poured out and solidified enormous portions of grey baked beans in blackish tomato sauce. Very British rock food. Here and there were clear borders between individual conglomerate beds, with varying pebble sizes. Some beds have pebbles in pebble size, some have larger boulders up to meter size.

More conglomerate and lavas at Crawton: The coarse boulder conglomerate, with lavas and basalt columns on the other side of the bay.

On top of the baked beans conglomerate is the pie crust, black and scorched – the lava. The flow is around 20-30 meters thick at Crawton. Much of it looks just like grey-black stiffened, fractured, goo – or fine-ground haggis. But there are also the classic basalt columns with their familiar six sides, some radiating out like flowers.

Walking on the top of the basalt flow, the basalt columns appear to be hardest at the edges, and slightly softer, at least as soft as rocks can be, in the center. The centers were often filled by ponds, creating modern art-by-nature.

Which sci-fi planet is this? The top of the basalt columns at Crawton look almost like some big organism, created by the centers being eroded, and the rounded edges standing out.

Every rock tells a story, we geologists like to say. It’s very true. But it may be even better to say that that every rock is part of a story. We can’t read the whole story of eastern Scotland 420-ish million years ago in a couple of outcrops. But they provide pieces in the puzzle, and together with many other outcrops, sediment logs, age datings, measurements of the direction of currents in those old sands, we can put together the bigger picture, of the the wild rivers in front of the rising mountains in the Silurian.

How to get there: The maps below show the location of Stonehaven and Crawton. Stonehaven is ca 25 km (16 miles, for those who stick to the imperial system) south of Aberdeen, ca half an hour drive. Then, ten more minutes (5 miles, 8 km) to Crawton.

Stonehaven: The images in this post are taken along the shore from Cowie in northeast Stonehaven, and along the shore towards Craigeven Bay. The hard, NE-SW oriented band of grey rocks are bent-up, deformed and hardened rocks along the fault plane of the Highland Boundary fault.

Crawton: This overview of Crawton shows the location of the bay with the waterfall in top. The lavas resting on conglomerates are on the rugged peninsula south of the bay; he NNW-SSE striking fracture lines mark a small gorge down to the shore.

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