Going underground #1 – flint and brick

Posted on 8 June, 2024 by Metageologist

The further you go from home, the more exotic things become, whether for holidays, or for an imaginary journey straight down. There are wonders under your feet, much closer than you think.

We’ll start from my house, in an unremarkable suburb of Reading, an almost-city in the Thames Valley, west of London, England.

Put a spade into the ground in my back garden and it sinks in gratefully. This is well-cultivated soil, dug for Britain during the war. The stones within it are mostly flint, occasionally fossils and mostly rounded. Immediately below the soil is a gravel terrace laid down by the River Thames.

A fossil sea-urchin, made of flint, from my garden

This gravel is extremely useful in construction and widely extracted. A map of Reading shows many bodies of water, dug for the gravel. The entire area is covered in these terraces, of various ages. The further up and away from the modern river you go, the older the terraces – they trace how the river has slowly cut its way into the landscape.

My house is not so close to the river, so the gravels are about 250 – 300,000 years old, a time a few ice ages past, when humans had learned to make fire and stone tools. A gravel pit behind my house unearthed some of these tools, now in Reading museum.

We’ve travelled just 6 metres. Below the gravels we get the first ‘solid’ geology – the London Clay and Lambeth Group, layers of sand, silt and mud around 50-60 million years old. Dramatic things were happening at this time – huge volcanoes formed in the west of Scotland, India first collided with Asia to form the mighty Himalayan mountains. In SE England alas, not much was going on, just a bunch of mud and sand being draped over the surface.

What’s most interesting about these rocks is what people did with them. Particular layers are good for making bricks and this has shaped the local area. Reading town centre is full of fine Victorian brick-buildings. The local clay gives red bricks, but mixing in quantities of chalk gave a grey coloured brick. Bricks were made across the Thames Valley, wherever suitable clay was found.

Lidar image of Nettebed. The smooth dry valleys in the top half are chalk, the pitted hill is where the clay was dug

BGS map of the same area. Green is chalk, brown/purple contain clay

Unlike older, more deeply buried sediments, these rocks are still soft and poorly bound together, something that led to the first ever serious railway accident. The Great Western Railway was originally planned to run through the village of Sonning, close to the Thames. However an uncooperative land-owner forced the route to go through a small hill. Any plans to tunnel through were dashed by the low strength of the sediments. Instead a cutting was made – a 17m deep gash through the hill – with the railway running at the bottom of it. On the 24th December 1841, a landslip covered the rails with ‘soil’. A train ran into the slip, causing the deaths of 9 people. These were early days for both railways and engineering geology – the cutting was later widened, making the slopes less steep.

Only 24m below my house, we reach the Chalk. This is a rock-type that is both an icon of Southern England (“white cliffs of Dover”) and deeply weird.

While most sedimentary rocks are made of sand or mud, chalk is almost entirely made up of the remains of living creatures. Sea-levels at the time (95-65 million years ago) were extremely high, meaning much of Britain was underwater. Since little or no sand or mud was being washed into the sea, the only things settling on the sea-bed were the remains of tiny planktonic organisms. These grew tiny beautifully shaped flakes of calcium carbonate that found their way to the sea-bed, building up layer after layer. Other creatures, like sea urchins living in this chalky mud had parts made of silica. These formed into nodules black flint, so prized by our distant ancestors. Rare stones are found in the chalk, thought to have polished in the insides of giant sea monsters, marine reptiles that swam these seas.

Chalk landscapes form the bones of southern England. Slightly harder, they form ridges such as the Chilterns and South Downs. These are strange landscapes, dotted by prehistoric structures and mysterious figures formed by cutting through the thin soils to reveal the pure white chalk beneath.

These chalk hills have sinuous valleys and whale-backed hills. The valleys are dry – chalk is a rock that swallows rivers and is in turn dissolved by water. Near me, above the chalk, a swallow hole recently appeared in a school car park, a reminder of the chasms beneath our feet.

There are creatures beneath my feet. The chalk aquifers are home to creatures called stygobites, ghostly crustacean that eke out a living in wet cracks. Some are thought to have been living in England for up to 19 million years, making them our longest continuous inhabitant

The remains of more ancient life lies deeper still. As we shall see, next time.

Assynt’s etched landscape

Posted on 11 October, 2022 by Metageologist

Some place names describe the shape of landscape. South east of Lochinver lie Cnoc a Mhuilinn (“Mill Hill”), Gleann Sgoilte (“Cleft Glen”) and Gleannan na Gaoithe (“Windy Glen”). These are dramatic features where the land has been cleaved, leaving narrow slots where the wind howls and narrow fast rivers make mill streams.

These dramatic features are all aligned in space. How did they form? A paper from 1956 maps them out and links them to a now vanished lake they call ‘Loch Suilven’. Using “an
ex-R.A.F. bubble-sextant in conjunction with a box-sextant“, Alex J. Boyd of Inverkirkaig carefully mapped the fossil beaches left behind by this Loch. From this work it is clear that water levels were once much higher. He inferred that the lake once drained via our dramatic glens, until the modern Inverkirkaig river breached the lake and drained it, separating it out into Fionn Loch, Loch Veyatie and others.

Location map. Figure 2 from Boyd (1965).

The picture is complicated by ice. This landscape is haunted by the Ice Age and any carving into the ground could have been done by ice as well of water. Glacial melt water channels are features formed by the flow of ice close to glaciers. Water beneath may flow under pressure, or ice may dam rivers, or melt suddenly to create large volumes of water. Under these conditions, channels may be cut by catastrophic flood events.

Tracing the western edge of the area, around Badnaban (place of the women) Boyd reports “rounded boulders” of large size that suggest the water flow was of “torrential character”. High up, the walls of the channel below Cnoc a Mhuilinn have a polished appearance with scalloped shapes. The British Geological Survey interpret these as S-forms and P-forms and call the structure a glacial meltwater channel.

View of a rock surface in the glen below Cnoc a Mhuilinn

A brief scan of modern literature suggests that there is no consensus about the precise meaning of these features, but for sure they are consistent with a dramatic flooding event scouring out the channels. Ice flow may also have shaped the glens and both mechanisms may have acted at different times. But maybe what was eroded is more important than how it was done.

Glacial meltwater channel below Cnoc a Mhuilinn

The sculpture within the block

There’s a quote attributed to the sculptor Michelangelo, to the effect that the sculpture is already there within the block of marble, his job is merely to remove the material surrounding it. For this landscape, this may be literally true. We’ve talked of ice and water cutting into the ground, but maybe the most important features were already physically present within the bed rock, waiting to be revealed.

The cnoc and lochan landscape is an extremely distinctive feature of Lewisian gneiss areas. A similar landscape exists in Connemara on the deformed igneous rocks of Roundstone Bog. The writer Tim Robinson describes this landscape as “frightened”. It’s a largely random distribution of lochs and hills, straight lines appearing where it is cut by geological faults, breaks in the rock that weaken it.

Maarten Krabbendam and Tom Bradwell (of the British Geological Survey) have reviewed the cnoc and lochan terrane in Sutherland and conclude that its distinctive roughness is not a direct product of glacial erosion.

They review similar landscapes in other countries and note that they are a feature of rock type, not of erosion style. They compare the Assynt area with similar terrane in Namibia, where erosion is caused by wind-blown sand rather than ice. They show that the shape of the landscape is mostly controlled by chemical weathering, that weakens solid rock into weak ‘saprolite’. The role of erosion, whether scouring by ice or glacial meltwater, is to remove the saprolite, leaving the harder rock behind.

Chemical erosion is highly sensitive to rock type. The Lewisian Gneiss is cross-cut by vertical sheets of rock called dykes. One particular dyke (400 million years younger than the other Scourie dykes) is rich in olivine, a mineral too chemically delicate to last long at the surface. This dyke easily rots into saprolite, forming deep zones of soft rock. The glacial meltwater channel below Cnoc a Mhuilinn precisely follows this dyke; its location was determined not by the ephemeral flow of ice or water, but by events 1,992,000,000 years ago. Gleann Sgoilte was cut into another dyke and Gleannan na Gaoithe formed along a later shear zone, full of fractured and altered rock.

This pattern is common across the whole of Assynt, wherever Lewisian Gneiss is found. These remarkable glens may be more dramatic than most, thanks to glacial floods, but these were simply picking out the bones of the rock. This landscape was etched rather than scraped.

Looking out for the red rocks

Posted on 31 August, 2022 by Metageologist

Author Tim Robinson spent countless hours in the west of Ireland, unearthing local Irish-language place names. Some are anchored in myth and poetry, referring to miracle-working saints or Celtic Gods. Most though are prosaic, being linked to people’s names, local plants or animals and – occasionally – geological features.

Fàire nan Clach Ruadha is one of the countless hills within Assynt’s central wilderness, just another small summit amid the craggy wastes of the Cnoc and Lochan landscape. Once I’d roughly translated it as ‘red stone lookout’, I knew I had to pay a visit. Secretly I was hoping the ‘red rocks’ would be garnet-rich, as is often the case within the Lewisian Gneiss.

The sort of red rocks I was hoping for. Garnet-rich Lewisian Gneiss from Scourie

On a perfect June day I went in search of the physical realities behind the name. Let’s start with ‘Fàire’. The online resources I’ve used translate this as ‘watch, lookout’, which implies someone once sat on the top of the hill, keeping an eye out for something. This could have been livestock: sheep or cattle. Subsistence farming practices saw animals brought into the interior during the Summer, to make use of the good grazing found here and keep them away from arable crops. Ruins out here are often Shielings, simple buildings built for Summer use only. We are not far here from some and the hill offers good all-round views of the immediate area.

So maybe previous visitors were enjoying themselves, having a relaxed Summer just like me. Tim Robinson translates the Irish phrase for a nostalgic return: “Cuairt an lao ar an athbhuaile” as “the calf’s visit to the old milking place”, a reference to the practice of taking livestock into the hills in Summer. This booleying (to use the Irish terminology) lasted until the 1930s in Ireland and Robinson quotes some idyllic sounding memories of those times.

View west from Fàire nan Clach Ruadha, out to sea.

There is another possibility. Walking inland, this hill is the first to give really distant views of the sea, to raise you above this land’s crinkly corrugations. In the photo above you can see the Achiltibuie peninsula, top left, about 10 km distant. The faint smudges of the Outer Hebrides are clearly visible to the naked eye. Maybe this was a place to look out for ships. There are Norse-inflected names are present near here – Suilven and Inverkirkaig for example. Could a lookout here once have run off in terror, sprinting to give advance warning of a hostile Viking attack? On a glorious June day it seems like an impossible idea. Surely a dot on the horizon would cause joyful anticipation of a returning loved one, home from the sea.

What of the “red rocks”? While the bedrock of Fàire nan Clach Ruadha is made of Lewisian Gneiss, blocks of red Torridonian sandstone are found in great abundance here littering the surface. Some sit proud on the top of the hill, but the most abundant areas are on west-facing slopes.

As for much of Assynt, photos taken from here become dominated by the great charismatic peaks of Suilven and Stac Pollaigh. Like being photo-bombed by a celebrity, they immediately become the focus of the image, demanding your attention through sheer charisma. These peaks are also made of Torridonian sandstone, so on Fàire nan Clach Ruadha it feels like they are proud parents, peering down upon their tiny offspring.

Suilven peering down at its little baby mountain

Imagine this is how mountains form. The big parent mountain silently urging its offspring to grow up big and strong. Perhaps they are like sharks, the biggest feasting off their siblings in a race to reach adult size. I could be in the midst of a massacre, too slow to register on human timescales.

Of course this is actually how mountains die. Each block was plucked from the side of Suilven by ice and left behind by a now vanished ice sheet. We know this, as people have laboriously mapped the location of these ‘boulder trains’ of Torridonian sandstone, showing a clear link.

Image from Lawson (1995).
Fàire nan Clach Ruadha is within the boulder train coming from Suilven

This academic paper ends with a great acknowledgements section, thanking “the unstinting, though often forced, efforts of a large number of A level Geography students ….helped in the plotting… across this knurled and unyielding landscape, often in the most unhelpful of weather“.

Place names are echoes of how past generations have engaged with a landscape, a reminder that our feet are not the first to tread these rocks. Subsistence farmers on a lazy Summer’s day; somebody anxiously scanning the sea; wet and grumpy teenagers; maybe all have noticed these red rocks before me.

Change & Lady Constance

Posted on 20 July, 2022 by Metageologist

Inspired by reading Tim Robinson, I’m very interested in the idea of delving deep into a particular place and all the stories it can tell. Tim Robinson sought to know everything about the places he lived, taking in geology, archaeology, history, language, place names and more. He lived in the west of Ireland for most of his life and produced numerous books. I’ve been on holiday and written a few blog posts – a few ripples in comparison to the great splash of his work – but I hope they resonate harmoniously.

I’ve written about an area just south of Lochinver in the west of Scotland. This first post is about two places and why Assynt is so empty.

Hill of the horses

Cnoc nan Each – “Hill of the horses” is both a hill and a place – shown on the OS maps as Cnocnaeach. Fewer than 2 kilometres from the pie-munching tourists of Lochinver it could not be more different.

View North over Cnocnaneach. Ruined building in the foreground. No trees, no people.

It’s an attractive spot, a flat-bottomed area underlain by sediment and a contrast to the craggy wastes surrounding it. A range of different ruins show that people lived here for thousands of years. A range of structures (well described here and here) attest to countless generations tending livestock, making a living off the land.

All are gone. The gaelic-speaking people – who lived here and named every hill – were “cleared away” in the Nineteenth Century, put onto ships to Canada, all in the name of ‘improvement’. In economic terms, subsistence agriculture made no sense. To the land-owners, removing the people and filling the land with deer and sheep was a rational decision. The people who were cleared saw this as a tragedy, but nobody cared what they thought. Nobody with power, anyway.

Lady Constance Bay

Just 3 kilometres west of Cnoc Nan Each lies a small bay, marked on some OS maps as ‘Lady Constance Bay’. It’s named – I infer – after Lady Constance Gertrude Sutherland-Leveson-Gower, a member of the British aristocracy. At 18 she married the man who became both the Duke of Westminster and the richest man in Britain.

Constance is unlikely to have spent much time in Lochinver, but as the daughter of the 2nd Duke of Sutherland, she does have a strong connection to the place. Her family were descended – via the Scottish and then British aristocracy – from the head of Clan Sutherland. In Medieval times a clan head was a warrior chief, but by the Nineteenth Century the role was one of absentee landlord. Constance’s grandmother – as owner of most of Sutherland – ordered the clearances there in the name of improvement. The interior of Assynt was depopulated in 1812 on her command.

By 1847 the land was largely empty. In that year, Constance’s father – who liked to spend his money on improvements to his many buildings – planted Culag Woods. This attractive piece of woodland just south of Lochinver is clustered around Cnoc na Doire Daraich or “Hill of the oak thicket”. Lady Constance Bay lies just offshore the woods. Constance was 14 at the time; the bay was surely named in her honour.

New beginnings

Culag woods remains. It’s a community woodland and clearly well loved. The name Lady Constance Bay is not shown on the maps of the wood. I don’t know if the omission is deliberate, but it seems appropriate. Constance’s descendants remain some of the richest people in Britain, but they no longer own Assynt.

Change is slowly coming to Scotland. These two places are still on land owned by absentee billionaires, but following a community buyout a large portion of the interior of Assynt is owned by the Assynt Foundation, to be managed on behalf of local people. A fascinating aspect of the Highlands are the debates over how to move on from the ecological and human impacts of the clearances. Rewilding, wind farms and community buy-outs are all things bringing change to the area, but each raise as many questions as answers. As an outsider, it’s not for me to say how things should change – I’m not even clear whether my holidays there bring benefit or harm – but I’ll be watching with fascination.