Geologists uncover the secrets and symbolism of 100 million years old immense block of chalk in England’s landscape
On the British Geological Survey’s map, chalk is represented by a swathe of pale, limey green that begins on the east coast of Yorkshire and curves in a sinuous green sweep down the east coast, breaking off where the Wash nibbles inland. In the south, the chalk centres on Salisbury Plain, radiating out in four great ridges: heading west, the Dorset Downs; heading east, the North Downs, the South Downs and the Chilterns.
Stand on Oxford Street in the middle of the West End of London and beneath you, beneath the concrete and the London clay and the sands and gravels, is an immense block of white chalk lying there in the darkness like some vast subterranean iceberg, in places 200 metres thick. The Chalk Escarpment, as this block is known, is the single largest geological feature in Britain. Where I grew up, in a suburb of Croydon at the edge of south London, this chalk rises up from underneath the clays and gravels to form the ridge of hills called the North Downs. These add drama to quiet streets of bungalows and interwar semis: every so often a gap between the houses shows land falling away, sky opening up, the towers and lights of the city visible far in the distance.
The British Geological Survey (BGS) was established (as the Ordnance Geological Survey) in 1835. The world’s first national geological survey, its original remit was to survey the country and produce a series of geological maps. Today, the BGS, which still produces the “official” map of the UK’s geology, is best described as a quasi-governmental organisation split between research, commercial projects and “public good”. Quite a lot of its work is now done outside Britain: at the time of writing, projects include studies of groundwater in the Philippines and volcanic activity in the Afar region of Ethiopia.
One week in early October, four members of the BGS set up camp in a self-catering cottage near the town of Tring in the Chiltern Hills, about halfway between London and Oxford. They were on a training exercise as part of a project to produce a new geological map of the chalk of southern England. On the day I arrived, the wooden table in the main room was covered with maps, books, a half-drunk bottle of red wine and a packet of chocolate digestives. Field leader Andrew Farrant, tall and thin, with steel-rimmed glasses, was drinking a cup of tea. He had a sort of leather holster attached to his trousers, from which swung a geological hammer with a surprisingly wicked-looking long, pointed end.
Farrant has been working on the chalk-mapping project on and off since 1996. “I would say that not enough attention is paid by the academic research community to understanding the geology of the UK,” he said. “If I was doing this [mapping project] in east Greenland, then I’d probably get funding for it – east Greenland is sexy. And people tend to think that because we have a geological map of the UK, it’s all been done, but actually you can still improve it.”
The geology of the Chilterns, for example, was last mapped in 1912. Since then, the discipline has changed quite a bit. Geologists now know about plate tectonics and radiometric dating. There are laser-based distance measurements for elevation maps and digital terrain models and higher-definition Ordnance Survey maps, allowing hitherto unrecognised features to be recorded. All of this will affect the maps that are produced.
And when it comes to the chalk, these new maps matter in a way they didn’t in 1912, because since then, the population of the south-east has increased by roughly a third. In particular, this jump has put pressure on the region’s transport systems – often created by tunnelling though chalk to form such projects as HS2, the Gravesend tunnel and Crossrail – and the region’s water resources, much of them stored in the chalk aquifer.
Imagine stumbling, blindfolded, through an unknown landscape, uneven terrain underfoot, and large, hard objects rearing out of nowhere. Without decent mapping, this is essentially the situation for a tunnelling engineer faced with an immense block of chalk. “Obstructions are a very big issue,” Mike Black, Transport for London’s principal geotechnical engineer, recalled in an interview in New Civil Engineering. “We spend a huge amount of time on desk studies trying to work out where everything is or where it might be.”
An unexpected flint band or hard rock stratum can shatter the shield of a £100m tunnel-boring machine. Hit a fracture or a seam of clay, and your tunnel – filled with men and machines – might flood with water. The Channel tunnel, for instance, doesn’t go in a straight line from A to B, but follows as much as possible a single layer in the chalk that is one of the most suitable for tunnelling. To plan the route, engineers looked at samples of chalk from boreholes and analysed the microfossils in order to find the best way through. “That saved Eurotunnel probably half a million pounds,” Farrant told me.
The world’s first true and comprehensive geological map of a country – England, Wales and (most of) Scotland – was published in 1815, by a surveyor named William Smith. In an age of gentlemen geologists, Smith wasn’t rich, posh or well-connected – in fact, his social status barred him from membership of the Geological Society of London – but he was obsessed with rocks, fossils and the idea of mapping the geology of Britain. He spent years travelling around the country to gather material, eventually bankrupting himself while producing the first copies of his map.
Today, one of the original copies hangs in the entrance hall of the Geological Society’s headquarters in Piccadilly. When you pull back the blue velvet curtain protecting it from the light, one of first things that strikes you is its beauty. The UK is furrowed by a series of curving lines running downwards right to left to reach a point around Taunton in Somerset. The country is a marbled mass of forest green, caramel brown, bubblegum pink, rich purple and pale lavender.
Looking at Smith’s map, you can tell at a glance that the country is older in the west and younger in the east; that, roughly speaking, if you begin in the south-east and travel north-west up to the Highlands of Scotland, you travel back in time – from the newest formations of East Anglia to the ancient metamorphic rocks of the Highlands. Smith gave each stratum a different colour, based loosely on the colour of rock they indicated, and graded so that the strongest colour represents the base of the formation, lightening upwards.
The colours Smith chose are, more or less, those still employed by all stratigraphers today. They are based on the colours of the rocks themselves: yellow for the Triassic sandstone of Shropshire, formed from hot, dry deserts; pale pink for Cambrian granites extruded from prehistoric volcanoes in what is now Wales; blue for the coal-bearing Carboniferous rocks of the Midlands, when that region was a land of seething, glistening swamps; pale, yellowish green for the white chalk, because white would have shown up badly against the paper.
Smith’s map helped to shape the economic and scientific development of Britain during the Industrial Revolution. It showed where coal to power the factories might be found. Where clays and rocks to build the growing cities might be quarried. Where tin and lead and copper could be mined. Where a canal or railway line might most easily be dug. His map represented an increase not just in knowledge, but also in wealth.
Smith is sometimes known as “the father of English geology”. In 2003, one of his original maps was sold for £55,000. In Piccadilly, the society that would once have refused him membership displays his relics like those of a saint: an oil painting complete with a lock of Smith’s white hair sealed into the frame and two uncomfortable-looking wooden chairs.
The study of chalk is what is sometimes termed “soft rock” geology. Soft rock experts study “sedimentary rocks such as sandstones and limestones, while their “hard rock” counterparts work on the tough igneous and metamorphic rocks such as granites and slates. The categories aren’t perfect, but the jargon sticks. Rivalry sometimes ensues. I once met a retired sedimentary geologist who argued that “soft rock men” are always the more thoughtful. It came, he mused, from thinking about the formation of sedimentary rocks. One rock unit formed from the quiet accretion of layers of sediment over many millions of years. The slow, slow formation of worlds. And what about hard rock geologists? I asked him. “Hard rock men are all bastards,” he said.
The chalk world began to come into existence around 80-100 million years ago, when the Earth was entering a warming phase. Seas rose rapidly, and one third of the landmasses present today disappeared beneath the rising waves. Geologists call this period the Cretaceous, after creta, the Latin for “chalk”, and it is the longest geological time period on the stratigraphic chart: at 80 million years, it lasted far longer than the 65 million years that have elapsed since it ended.
In regions where chalk is found today, the water was filled with billions of microscopic organisms called coccoliths. When they died, their skeletons sank down through the clear water, in such quantity that in places the ocean would have turned a milky blue. On the ocean floor the skeletons piled up, forming a soft ooze. Over time, this compacted and hardened – living bones translated into white rock. The uniformity of the chalk – these massive thicknesses of rock, some a mile in depth – is testament to a stable, slowly drifting world where, for millions of years, nothing much happened.
During the late 19th century, geologists began further refining the existing rock units of type and time. But relatively little attention was paid to chalk. Geologists felt there wasn’t much to say about it, and little economic imperative to study it in greater detail. It had some use as a fertiliser and, later, in concrete, but it contains no coal, oil, precious minerals or metals, and is generally too soft to be a building material.
Even among that subsection of the population who get excited by a good piece of rock, for years chalk was seen as fairly dull. When Farrant started work at the BGS in 1996, he told me: “I got dumped on the chalk and I thought, ‘Oh God, how boring.’ It turns out I was wrong.”
In Britain – or, more accurately, the place that was to become Britain – the next big thing to happen to the chalk occurred about 50 million years ago, when the African plate crashed into Europe. The land buckled up, forming a series of ridges including the Pyrenees and the Alps. In Britain, a series of low chalk hills began to emerge from the sea. At first they were capped with mud and sandstones, but erosion eventually did its work and formed the bare chalk scarps of the South and North Downs and the Chilterns.
Today in the south-east of the UK, much of the chalk has disappeared underneath sprawling towns and suburbs, but where it hasn’t been built over it produces a landscape often viewed as quintessentially English. Smooth, rolling hills covered with short turf. Gentle slopes and steep escarpments, dry valleys and lonely beech hangers. Seen from a distance, it seems to ebb and swell like the ocean from which it once emerged.
On postcards and tea towels, images of chalk landscapes perform a particular version of Englishness. “Chalk has quite a central place in England’s cultural history – the white cliffs of Dover and all that stuff,” Farrant said. “And yet most people know nothing about what it is and how it formed.”
At the edge of the country, the chalk becomes dramatic, unsettling. Standing on the beach at Cuckmere Haven in Sussex, you look up at the towering whiteness and it seems for a moment as though it is falling towards you out of the blue sky. The exposed chalk has something cold and otherworldly about it. To see such whiteness, such brightness, feels unnatural.
From the south coast, the chalk runs underneath the Channel and reappears as another set of white cliffs, which the French call the Côte d’Albâtre (“Alabaster Coast”) and the English tend not to talk about very much. These were much painted by Monet, Pissarro and Renoir. Chalk, which the English often seem to regard as peculiarly their own, lies under much of northern France, and bits of Scandinavia, the Netherlands and Germany.
In 1993, Richard Selley, then a professor at Imperial College London, had been thinking about the similarities between the chalk landscape of the North Downs and the Champagne region in north-east France. His neighbour had been unsuccessfully trying to farm sheep and pigs on his estate in the North Downs. Selley suggested he try sparkling wine. That vineyard now produces close to 1m bottles of wine a year, about half of it sparkling – which would, if made in north-eastern France, be called champagne.
As Farrant said: “The English Channel is really a minor thing. It’s the same deposit basically, so there’s no Brexit with the chalk.”
The Chiltern Hills run for 46 miles from Goring-on-Thames in Oxfordshire north-west to Hitchin in Hertfordshire. At their highest point – Haddington Hill in Buckinghamshire – a stone monument marks the 267-metre summit. Much of this landscape is farmland. There are small villages huddled deep in the dry valleys, historic market towns and the edges of suburbia. I joined Farrant and his BGS colleagues there on a warm day of blue skies and strong, low autumn light. Farrant and I set off with a new recruit called Romaine Graham, who had been working on the chalk for two weeks and had blood blisters on the palms of her hands from wielding her hammer.
We followed a track between hedgerows full of fat, red rosehips and rambling old man’s beard. We climbed over a barbed-wire fence between two ploughed fields; where there are no footpaths, the surveyors rely on the goodwill of landowners for access. Farmers are usually OK, but gamekeepers tend to be territorial. By the edge of the field, Farrant and Graham used their hammers to break open pieces of chalk. “This is the Zig Zag Chalk,” Farrant says. “It’s medium-hard, pale grey and blocky.”
We know now that the chalk was never just the three large, monolithic blocks of rock (and time) that the 19th-century geologists proposed – Lower, Middle and Upper. In the 1980s, geologists began subdividing the chalk into nine formations. As we walked, Farrant and Graham began to discuss differences between formations. To the uninitiated, these can seem negligible. Working in chalk is all about getting your eye in, reading the subtlest of clues. The Zig Zag, for example, they described as “rather dull, John Major grey”. The Seaford, by contrast, is soft, smooth and bright white, and often contains large flints. The Holywell is creamy white, filled with small fossils. The Lewes is white, creamy or yellowish. Chalk rock is very hard, closer to the hard limestones of Cheddar Gorge than the soft, crumbly white stuff that most of us think of as chalk. Each formation represents a different world, and each of these worlds existed for far, far longer than humans have been on the planet.
Where there are not many outcrops, the surveyor must find other ways of getting at the chalk. You look for old quarries and pits, badger setts, newly ploughed fields, or even graveyards, where the earth has been recently turned. Working on a site at Stonehenge, Farrant found himself on his hands and knees looking for molehills beside the roar of the A303. “Doing this work has got harder recently,” he said. “Over the past 10 years farmers stopped deep ploughing. Now they use something called a no-plough method, where they just put the seeds straight in the ground, which is fantastic for wildlife, but for us it’s a right pain.”
Up ahead he spotted a small copse, which he thought might contain the remains of an old chalk pit, and dived into the undergrowth. “We spend a lot of time fighting through bushes,” Graham said. “Andy loves it.” By the time we caught up with him, he was sitting in the middle of the undergrowth hacking at a piece of chalk. “Totternhoe Stone,” he confirmed.
Mapping the chalk also relies heavily on what Farrant calls “landscape literacy”: the ability to determine what is underground by studying the surface. That might mean knowing that rounded hilltops are typically Seaford chalk, and flat fields typically Zig Zag. Or that where chalk is at the surface you find beech, yew and holly, and where it is deeper there are pine trees, heather and gorse.
By early afternoon the light had changed, and the fields glowed lavender and apricot. Up close the soil was light grey and dry, and the surveyors’ footprints looked like they were on the moon. The hill of Ivinghoe Beacon loomed above us – once the site of a bronze-age barrow, then an iron-age fort – rising up abruptly from the Vale of Aylesbury to form part of the ridge of the Chilterns.
As we began to climb, we passed an exposed bank of chalk, created when the path was cut into the hillside. Here the surveyors thought they might find fossils. Graham lent me her hammer, and after five minutes we’d amassed a small collection of long-dead sea creatures. Pieces of chalk split in half to reveal a brown tubular worm, a brachiopod shell like a toenail, and the perfect spiral of an ammonite.
“What I really love are trace fossils,” Graham said. “They can tell you so much.” Trace fossils are the remains not of the creature itself, but of its footprints, tracks, burrows or faeces. “Sometimes you can see two tracks – maybe two trilobites skittering across the sand – and you can see where they join together for a bit, have a little party. It’s easier to think about past landscapes when I can see traces of the creatures that lived in them. You think, wow, it was literally here.”
Building on techniques pioneered by Smith in the early 19th century, modern surveyors use fossils and microfossils to identify layers of chalk. Back in 2002, Farrant told me, police called on the help of local geologists when a tiny fragment of chalk was found underneath the wheel arch of the Soham murderer Ian Huntley. Two particular microfossils were discovered in the chalk: one found only in the Seaford and one only in the upper Newhaven. The presence of both microfossils meant that the chalk fragment could only have come from a specific 2-metre thick layer – and the only place that chalk could have been driven over was a local farm track that a farmer had covered with that specific chalk, and where Huntley claimed he had never been. The chalk fragment formed part of the evidence that eventually secured his conviction.
At the top of the Beacon we sat down. It was very still and very silent. Somewhere up above a skylark was calling. From here you could see the fields of Buckinghamshire, Bedfordshire, Hertfordshire and Oxfordshire beyond. In the distance a row of small bushes flamed yellow and red, a line of fire along the edge of the green field. It made sense, I was thinking, that the first people to live here headed for this place, climbed up the hill to where a view of the world opened out.
Graham ate a banana and said that tomorrow she wanted to try to collect sloes. “It’s not always as pleasant as this,” Farrant warned me. “You should come back when it’s a freezing, raining day in January and we’re stuck surveying some industrial estate in Watford.”
He got out a laptop and began to enter data. The map they were working on is funded by the Environmental Agency and two major water companies. Because chalk is highly permeable, it acts as a huge aquifer, providing a source of drinking water. The chalk also acts as a natural filter, purifying the water that drains through it. But there are also fractures in the rock – and here the water flows instead of drains. Water companies need to know how the water flows through the chalk, where it can be safely extracted. And for that they need an accurate, detailed map of the different formations. The Holywell fractures in a different way from the Seaford. A crack in the Newhaven is not the same as one in the Zig Zag.
When he’d finished with his laptop, Farrant pointed downhill. “If you stood here during the Anglian Glaciation you would have seen an ice sheet coming right up to the base of the chalk scarp there.”
The next chapter of the story of the formation of the Chilterns took place around 450,000 years ago, when immense ice sheets covered the north of Britain, reaching down as far as Watford. Beyond the ice, the Chilterns was a wild expanse of cold, empty tundra. Unable to permeate the frozen ground, melting water flowed over the surface of the land, forming river channels that eventually cut down into the rock to create the dry valleys that are such a distinctive feature of the chalk landscape. “The whole of southern England has been beautifully picked out by that periglacial weathering,” Farrant said. Further north, everything was just bulldozed by the ice. I pictured great blocks of ice moving remorselessly across a landscape – ice heavy enough to grind and smooth away the very rocks in its path.
Afew weeks after my trip to the Chilterns, I went for a walk on the North Downs, on the other side of London. Following a farm track towards the ridgeway, the buzz and roar of the M25 was faint but insistent, like the distant rush of the ocean. Underfoot the path was pale brown and, where the thin topsoil had blown away, bright white – the bones of the land exposed. Reaching the ridge, I paused, turned and saw London in the distance. Grey and silver towers coming up out of a muzzy blueness away over the beech trees and red-tiled suburban roofs.The Anthropocene epoch: have we entered a new phase of planetary history?Read more
As I stood there, looking back towards the city, it seemed as though the blueness intensified. And then it looked for a while as though the old Cretaceous ocean had returned to the London basin. Or as though I was seeing a flooded city some time in the future. I thought about melting ice sheets and sea level rise and how, as I stood there, the south-east of the island was sinking while Scotland rose up – a see-saw effect caused when the great northern ice sheets began to melt around 20,000 years ago.
And then, I imagined, the ground in the city would become heavy like a saturated sponge, the groundwater seeping up between the paving stones, bubbling up out of the drains and running along the gutters. The Thames would swell and over-top its banks. Fingers of brackish water creeping up Cheapside and into the grounds of St Paul’s. The water rising over the Houses of Parliament, Big Ben, the Palace of Westminster. A blueness overtaking the landscape.
Courtesy: The Guardian