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Urban geology

You don’t need to set off on a field trip to find stones that are older than the human race

Helen Gordon | September/October 2014

Every weekend, as the buses rumble down Regent Street, people pile into the Apple Store to spend their savings on the thrill of the new. If they were to look down on their way in, they would feel the shock of the old. The pavement is made of grey sandstone, engraved with fine bands of curving lines—the ripples from the current of a river that flowed through what is now the Pennines, then a region of swamps, dense vegetation, high mountains and wide-open flood plains. The ripples are from 300m years ago.

This was during a geological period known as the Carboniferous. Mammals and dinosaurs had yet to evolve. On the land, amphibians moved among tree ferns and horsetails. The unnamed river contained tiny rock particles. As it flowed, these were deposited on the river bed and over time, through processes of compaction and cementation, they formed the sandstone known as Yorkstone—London’s traditional paving material. When you stop and stare down at the sandstone, you have a glimpse of a world that existed long before humanity.

Urban or street geology, by contrast, is young. In the 1970s a geologist at University College London (UCL), Dr Eric Robinson, began exploring the idea that geology could be taught not just on far-flung field trips but through the buildings and streets of our cities. Britons may not often go to Norway, but they can find polished Norwegian Larvikite on shop fronts in their local high street. It’s an igneous rock, meaning that it was formed when magma, molten rock, subsequently cooled. It’s also rather beautiful: dark green-blue with iridescent feldspar crystals. In April I went to Waterloo station to talk to Dr Ruth Siddall, who worked with Robinson. She first met him in 1989, when she was studying for a PhD in plate tectonics at UCL. At the time, Robinson was a lone voice arguing that there was historical and cultural value in the serious study of the building materials that make up our cities. He was also publishing a series of geo-walks around London for students and members of the public.

“The stones of London”, Siddall said, “come from all over the world.” For much of history and in most places, people have tended to build their towns and cities using whichever rocks were closest to hand. This was never possible in London, which rises up out of a natural basin underlain by the chalk that crops out in the Chiltern Hills to the north-west and the North Downs to the south. Bricks were once made locally, but the clays on which the city is built, and the surrounding chalk, are too soft to make satisfactory building stones. Siddall had agreed to take me on a new geo-walk from Waterloo to St Paul’s. In Benugo, the café where we were having coffee, she took a picture of the countertop: “marble,” she said, “a metamorphic rock, possibly from Carrara in Italy.” Metamorphic is one of the three main categories of rock, along with igneous and sedimentary, and it is related to one or the other—its name means an igneous or sedimentary rock that has been altered through processes of heat and pressure.

Around us the sun shone strongly on the white walls of the station and the towering Victory Arch. Waterloo was constructed between 1900 and 1922, using London’s classic building material: a limestone from Dorset called Portland Stone, a sedimentary rock that formed on the floor of a shallow Jurassic sea around 145m years ago, when Dorset had a climate like present-day Barbados. Siddall showed me how the seemingly smooth surface of the rock is actually formed from millions of ooliths—tiny spheres of calcium carbonate moulded by tidal action—and broken-shell sand. Fragments of grey oyster shell are visible. First transported to London by sea, it was used by the architect Inigo Jones in 1619, and then extensively by Christopher Wren and Nicholas Hawksmoor in the construction of their churches. It’s what UCL itself, Whitehall, the British Museum and much else is made of. Its popularity derives from its accessibility, plentiful supply, ability to withstand the weather, and its being what is known in the trade as a freestone—it can be cut in any direction.

As we walked, Siddall took pictures and made notes for a catalogue of London stones she is compiling in cahoots with the Geologists’ Association, and for a series of geo-walks that she publishes online and leads in person. Two teenagers stared as we stopped to snap some granite at the base of an office building. “Doing this,” Siddall said, “you have to adopt a no-shame approach to looking like a complete geek in public.”

She used to work in what might be called geo-archaeology, studying the geology of Greek and Roman building stones. Now she is dean of student welfare at UCL, a position that allows her to pursue her fascination with urban geology, free from the restrictions of the geology department. Eric Robinson’s approach to geology, placing it within human frameworks and timescales, still doesn’t sit comfortably with the subject as it is studied in British academia.

On the South Bank, outside the Royal Festival Hall, we saw the oldest rock on our walk: a glassy black gabbro from Pretoria, 2 billion years old, which forms the plinth for the giant bust of Nelson Mandela, erected in 1985. Inside the hall, visitors milled around, banners advertised a performance by the Simón Bolívar Youth Choir of Venezuela, and we looked at slabs of Derbyshire Fossil limestone. It’s a remarkable purplish stone filled with lighter-coloured shapes: discs, rectangles, things that look a little like tuning forks, and other things that look like chrysalises. In their profusion they reminded me of the images of deep space captured by the Hubble telescope. They are, in fact, the fossilised remains of a grove of crinoids or sea lilies, creatures from Carboniferous times, around 50m years before the dinosaurs. “This is the best example of a crinoidal limestone”, Siddall said, “that I’ve seen anywhere in the world.” You can see more of it at Hermès in Bond Street, where the leftovers from the Festival Hall ended up.

Crossing Blackfriars Bridge, we talked about the American geologist who saw a video of Osama bin Laden preaching in Afghanistan and wrote to the Pentagon, saying he had recognised the rock face in the background and knew, pretty much, where the video had been filmed. Geologists tend to be specialists. Once, on a field trip to a chalk quarry near London, I asked the organiser which was his favourite rock. The group was silent until one of the men turned to me and explained in scandalised tones that I’d just asked the country’s foremost chalk expert to name his favourite rock.

I decided to risk asking Siddall the same question. “Granites,” she said. “They’re sort of like ice cream: the same basic recipe but a whole world of flavours.”

For granite, another igneous rock, the ingredients in the recipe are feldspar, quartz and mica. The colour ranges from a kind of oatmeal—the Cornish granite at the base of Waterloo station—to dark red and the polished salmon pink of Peterhead granite from Aberdeenshire, which forms the pedestal for the statue of Queen Victoria at the northern end of Blackfriars Bridge. When polished, it can look rather like one of those aspic dishes from a 1970s cookbook.

We stopped to watch a boat filled with tourists chugging down the Thames. “You know the other great thing about granite?” Siddall said. “It’s why dry land exists.” About 4 billion years ago, granite, because of its buoyancy, was the material that rose to form the continental crust—the land that Europeans live on. In Britain, it’s the stuff of stone kerbs.

During the 19th century, granite and other materials could be more readily carried to London from around the country, thanks to rising wealth and the development of the railways. Nowadays, building stones tend to come from farther afield. On our walk we found stones from Cornwall, Dorset, Aberdeenshire, the Scottish Highlands, Italy, Greece, Norway, Sweden, China, South Africa and Australia. Ever since the Romans, who transported marble to London, certain special stones have always been brought in from overseas. (To see a fine example, go for a drink, as we did, in the art-nouveau interior of the Black Friar pub, with its green, purple and ochre marbles.) But today more everyday materials, such as granite for kerbstones, come from far away too. As Ted Nield shows in a new book, “Underlands”, the low price of oil and of foreign labour (with all that implies regarding the chances of decent working conditions) now makes it much cheaper for a British firm to source granite from China or India than Aberdeenshire. This is something that troubles Nield and Siddall.

Partly it is about retaining the character of a building or area: a granite from India is not the same as a granite from Cornwall. More urgently, it’s an environmental issue. The farther we move our building materials, the more fossil fuels we burn. In the midst of a global warming that the vast majority of scientists link to an increase of carbon dioxide in the atmosphere, this cannot be sensible. To complicate matters, many of Britain’s rocks now lie beneath areas designated as national parks and cannot be easily quarried. The situation turns into a paradox: the beauty of the landscape is preserved in the short term, at the expense of the existence of the landscape, in its current form, in the future.

Earlier this year I went to a talk at the Geological Society, where a former president, Bryan Lovell, spoke about a climate event 55m years ago, known as the Paleocene-Eocene Thermal Maximum (PETM). Geologists studying the rocks can tell that massive amounts of carbon were injected into the atmosphere, and there was a sharp increase in global temperature. “The PETM event wiped out a lot of mammals,” Siddall told me. “Mammalian evolution had to start again after that, so we know that it’s not good for us mammals, a heating climate. Yes, the Earth will recalibrate and something else will appear, but it won’t be us, and that’s not a nice thought.”

“A blindfolded geologist entering a brand new town, unknown to him, will have information on the local geology just by [looking at] the materials used in the buildings.” So said Francesco Rodolico (1905-88), a mineralogist and petrographer. I came across the quotation in a paper in the Journal of the Virtual Explorer, and was struck by the contrast between a city such as London and Rodolico’s town, built of local materials. Yes, London has its traditional building stones but a geologist set down on the South Bank might struggle to explain the local geology using the Festival Hall. Soon after my walk with Ruth Siddall, I travelled to Italy to meet one of the paper’s authors, Professor Vincenzo Morra, head of the department of earth sciences at the University of Naples Federico II.

Built on a slope, Naples rises up from the harbour towards green hilltops. Dim and narrow streets run between tall buildings, laundry hangs from windows, scooters buzz every­where and passageways open suddenly onto dusty sunlit squares. Churches with plain façades reveal interiors of rich marble and gleaming gold and silver. The university, founded in 1224 and counting Thomas Aquinas among its alumni, occupies a series of imposing buildings in the historic city centre. It also contains the beautiful Royal Museum of Mineralogy (1801) with its rows of glass-fronted wooden cabinets filled with 25,000 specimens from around the world.

I met Morra in his office, which is white-walled and high-ceilinged. He stubbed out his cigarette in a lumpy grey ashtray: “made of lava”, he said, “from Mount Etna.” Some of his colleagues arrived and located the espresso machine, wedged among the piles of books. Someone mentioned the Rodolico quotation and all the geologists started talking about colours. A city built from local materials will often have a distinctive colour scheme. Rome is white and red—white from the travertine, a form of limestone, and red from the bricks. Florence is white (marble), grey (Pietra Serena, a sandstone) and green (serpentine). Naples is dusty grey and sandy yellow: Vesuvian lava, Piperno and Neapolitan Yellow Tuff.

Where the traditional stones of London were largely formed under water (Portland Stone, Yorkstone), the stones of Naples come from fire. They are igneous rocks, volcanic in origin. The Neapolitans, like the Romans and the Greeks before them, live in a landscape ruled by volcanic activity. This is fertile country, but unsettled. From the Museum of Mineralogy you can see the towering blue peak of Vesuvius, which last erupted in 1944. To the west, where Morra and his colleague Professor Alessio Langella live, is a largely flat, densely populated volcanic area of craters, hot springs and fumeroles called Campi Flegrei, the Fiery Fields (from the Greek phlego, to burn). “Probably more dangerous than Vesu­vius,” Langella explained cheerfully, “because the eruption is not happening in just one place.” He and Morra first met at secondary school in Naples. They were particularly amused by the fact that Langella’s house is situated over a red spot—an area rated as being at risk from volcanic activity. Later, Langella spoke of Italy’s need for more professional geologists. “Because we have landslides, we have earthquakes, we have floods, we have volcanoes—anything you are looking for, you can find here!”

A volcanic eruption is the release of magma at the Earth’s surface. The magma flows from a vent or fissure in the form of fiery sticky liquid, or lava, with a temperature of 700–1,200 degrees Celsius. This may be accompanied by explosive eruptions, which emit particles of solid rock, ranging from tiny fragments of volcanic ash to large blocks of broken rock. When Vesuvius erupted in AD79, the town of Pompeii was buried in a hot ash deposit up to six metres thick, killing most of the inhabitants. Walking with Morra and Langella through the centre of Naples, I kept thinking about how very domesticated all this fire and brimstone was. On a day trip to Pompeii you can see those famous casts of ash-buried bodies, but in the Museum of Mineralogy are a series of jaunty souvenir medallions moulded from the lava of later eruptions, and on the streets of Naples the residents go about their daily business walking over dark grey pitted slabs of Vesuvian lava.

In a quiet courtyard with busts of illustrious men set among palm trees and bright green vegetation, we stopped to examine a weathered grey column of the rock known as Piperno. “This”, said Morra, “is the product of the largest eruption in the Mediterranean for 200,000 years.”

The Campanian Ignimbrite super-eruption took place in the Campi Flegrei area 39,000 years ago. Beside it, the destruction of Pompeii and even Krakatoa pale into insignificance. An area roughly the size of Belgium was covered with a fast-flowing mixture of extremely hot ash, flying rocks and white-hot boulders as big as houses. “Materials from the blast have been found as far away as Russia,” Morra said. The rock it created, the Piperno, is found nowhere else in the world. It is formed from compressed light grey volcanic ash with fragments of black flattened scoriae (basaltic lava ejected from a volcano), also known as fiammae. A hard, heavy rock, it is used occasionally for facing buildings—it gives the Gesù Nuovo church its forbidding, fortress-like air—and more typically for portals and ornamentation. Sometimes, depending on the cut of the stone, the fiammae look like little black flames flickering over the surfaces of the buildings.

In the streets to the north of the university, we passed Roman walls with diamond patterns picked out in pink bricks and yellow stones, and excavated blocks of sandy-coloured Greek masonry. For thousands of years people have been building here, often using the same rock: Neapolitan Yellow Tuff. Compared with the South African gabbro, the tuff is a young rock, a mere 15,000 years old. Like the Piperno, it is unique to this area and composed of compressed volcanic ash, but it is the product of a later, smaller eruption. It’s also much softer, lighter and more easily cut—a good building stone, if protected from weathering—typically with a covering of yellow plaster that echoes the sandy colour of the stone itself. Where the plaster has fallen away, the tuff has a spongy appearance, filled with tiny holes that look like air bubbles. Near the Piazza San Domenico Maggiore, I ran a finger lightly across an exposed block and it crumbled like the wall of a sandcastle. Alongside the grand churches and palaces and busy piazzas, you notice broken masonry, graffitied walls, small chapels closed for some indeterminably stalled restoration project. “The most important problem for conservation in Napoli”, Langella said, “is finding the money.”

Producers of wine talk about terroir, the idea that the soil, topography and climate of the land imparts certain qualities to the grapes grown there. Ruth Siddall believes in a terroir for stones, and it seems to fit the architecture of Naples. Whereas London builders had to send away for stones, the Neapolitans simply dug downwards, taking the tuff from underneath their city and piling it back up above ground in the form of houses, shops and public buildings. They left behind an immense network of passageways and around 2,000 caverns, a second city deep within the tuff. Later, these underground spaces became, among other things, water tanks and rubbish tips.

Morra took me to meet a former student of his, Gianluca Minin, who is developing a partly abandoned complex of passages, the Galleria Borbonica. In the 1950s it was a city car pound, but Minin has opened the space to the public as a sort of fantastical museum, artwork, concert hall and adventure playground. As we walked through the tunnels, abandoned Alfa Romeos and crumpled Vespas loomed out of the darkness, all coated in a layer of fine powder, like something out of J.G. Ballard—a vision of the death of modernity. “In 2005 the government sent me to survey the caves here,” Minin said, “and I fell in love, as it is possible to fall in love with a woman.” He gestured towards a space where Neapolitans had sheltered from Allied bombing raids during the second world war. “I wanted to save them, to preserve the history.”

In their paper in the Journal of the Virtual Explorer, Morra and Langella wrote with concern about the declining use of traditional local building materials such as the Neapolitan Yellow Tuff. It’s a familiar story. If the landscape near Naples was to be preserved, the quarrying near Naples had to cease. Morra and Langella favour a limited return to quarrying in the region, using modern, less invasive techniques, to provide materials for restoring and building significant architectural structures. In London Siddall had worried about mat­erials being shipped from China. Near the Piazza Miraglia, Langella pointed out a new-looking grey column. “Lava, but it’s not local,” he said disapprovingly. “It’s from Mount Etna.”

Before leaving Naples, I walked back to the Piazza San Domenico Maggiore, passing a church with small, pinkish flowers growing high up near the bell tower. In the piazza, students with heavy boots and metal piercings sat on the paving stones, talking and smoking. The lava had a glassy aspect, polished by generations of pedestrians. How many generations? How many years? And how many more years since the limestone of Waterloo station had formed, or the gabbro of the Mandela plinth? My brain rebelled. I couldn’t even comprehend 39,000, not really. With a lifespan of 80 years, how can we grasp the concept of 300m in any meaningful sense?

Geologists talk about the concept of deep time, meaning geological time, the amount that has passed since the beginning of the Earth, around 4.6 billion years ago. If we use the 24-hour clock analogy, whereby a single day represents the passage of deep time, land plants don’t arrive until 9.30pm, and the dinosaurs turn up at 10.40pm. Our ancestors split off from the rest of the hominids at two minutes to midnight. At this point it’s easy to feel derailed. A sense of paralysis seems a reasonable response to the immensity of the world, the smallness of the individual, the relentless crumbling of bricks.

In Naples, sitting outside at a café on the piazza, I looked over my notes. “It was Rodolico who first talked about the relationship between geology and building stones,” one of Morra’s colleagues had said. “Here in Napoli now, we are building very strong relationships with architects and engineers to try to solve the problems of decay, of conservation.” Slowly I became aware of some people in white overalls, entering the piazza in twos and threes. They gathered in the centre and unfurled hand-painted banners. Someone brought out a video camera. I went over to ask what was happening. A woman explained that in 2013 the European Union had awarded a large sum to restore Naples’ historic centre. (€75m, I later read.) The crowd, mostly out-of-work art and architectural restorers, wanted to draw public attention to the project. They were excited but anxious. There were rumours of delays, bureaucratic incompetence, perhaps malpractice. “There is so much work to do,” the woman said, gesturing around.

My walk with Ruth Siddall ended on the steps of St Paul’s Cathedral. Above us was that calm white famous façade, made of Portland Stone. The main landing on the cathedral steps is white Carrara marble with inner panels of red and grey Swedish travertine. If you crouch down to study the travertine, you can make out ghostly white markings, a segmented, cone-like shape. This is an orthocone from Ordovician times, a marine creature related to the squid.

Only a tiny proportion of life ever becomes a fossil, and only a tiny proportion of fossils are ever seen by humans. The odds are vastly stacked against it. That a creature should have died with its body intact. That an exceptional event such as a violent storm covered it with sufficient layers of sediment. That the sedimentary rock was not squeezed and changed by pressures deep in the earth so as to destroy the fossil. That the sediment formed a particular sort of rock that humans later decided was a good one to quarry. And so I thought about how strange it was, and how unlikely, that this 440m-year-old had survived, lying quietly in a piece of rock that would be selected by someone in 17th-century Sweden to ship to London to form the steps of a new cathedral. That it would be part of Nelson’s funeral in 1806 and Churchill’s in 1965, of Charles and Diana’s wedding in 1981, of the Occupy protests in 2012.

In Thomas Hardy’s novel “A Pair of Blue Eyes” (1873) Henry Knight, trapped on the side of a cliff, confronts the fossil of a trilobite. “The eyes, dead and turned to stone, were even now regarding him…It was the single instance within reach of his vision of anything that had ever been alive and had had a body to save, as he himself had now.” Thinking about geology can mean grappling with the shocking sweep, the impersonal vastness of geological time. A fossil delivers a different sort of shock. The shock of the small, the personal: the story of a single creature somehow like ourselves, with a body to save.

As we studied the orthocone in the afternoon sun, tourists posed for photos on the steps. I thought of Knight gazing at the trilobite. “Time closed up like a fan before him. He saw himself at one extremity of the years, face to face with the beg­inning and all the intermediate centuries simultaneously.”

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