In the heart of London’s Square Mile, between the windows of a tapas restaurant, a 150m-year-old ammonite stares mutely at passersby. The fossil is embedded in a limestone wall on Plantation Lane, sitting alongside the remnants of ancient nautiloids and squid-like belemnites. It’s a mineralised aquarium hiding in plain sight, a snapshot of deep time that few even glance at, a transtemporal space where patatas bravas meet prehistoric cephalopods.
How often do you give thought to the stones that make up our towns and cities? To the building blocks, paving slabs and machine-cut masonry that backdrop our lives? If your name’s Dr Ruth Siddall, the answer to that question would be yesterday, today and every day for the foreseeable. Her passion is urban geology, and it turns out that the architecture of central London – in common with many places – is a largely unwitting showcase of Earth science through the ages.
“This is York stone,” she says, pointing at the slabs beneath our feet as we wander the pavement of Eastcheap. An e-scooter swishes past. “It’s a fine-grained sandstone, around 310m years old, quarried in the Peak District. It was once a prehistoric riverbed – you can still see the ripples in the surface – although to picture the world back then you need to imagine Sheffield looking like the Brahmaputra [river, which spans China, India and Bangladesh].”
I’ve joined Ruth, a distinguished geologist and very affable company, on one of the walking tours she offers around different parts of the capital. Her own enthusiasm for street-level geology kickstarted in Athens in the early 1990s where, post-PhD, she was tasked with cataloguing a collection of rocks from Greek ruins. “It was essentially a big pile of rubble,” she smiles, “but it was an absolutely fascinating project. It got me hooked.”
In the decades since – and drawing inspiration from her former colleague Eric Robinson, a pioneer of urban geology – she has seen her adopted home of London in a new light. For Ruth, the city’s walkways, shop facades and statue plinths aren’t merely civic structures. They have epic stories to tell, not only in terms of their social history but their material origins, too. “London is huge, but unlike some cities it has no local building stones of its own,” she says. “It’s basically in a basin of clay, so all the stones you see around us have had to come from elsewhere.”
It brings fresh meaning to the idea of a rock biography. Around 10 years ago, in partnership with a fellow geologist, Dave Wallis, Ruth helped to establish London Pavement Geology, a website and app that gives a free comprehensive list of sites of geological interest around the capital, and increasingly in other UK towns and cities too (my wishlist is currently topped by the lobe-finned fish suspended in Edinburgh’s Caithness flagstones). Her guided walks, offered through the longstanding tour company London Walks (search for “geology”), will this year run on a roughly monthly basis, starting in spring.
Over two hours, we come across sedimentary, igneous and metamorphic rocks from places as disparate as Australia, Brazil and China, each stone type having been selected for its aesthetic value by the architects of the day. Outside a travel agency, Ruth identifies serpentinite, a Cretaceous stone from the Italian Alps. A pillar outside a pub turns out to be made of smooth 290m-year-old larvikite from Norway (magma that cooled kilometres beneath the surface of the planet, and, conveniently, easy to wipe down after a heavy night). And we linger over the Monument column, its Portland stone base crammed with Jurassic oyster shells and pitted with prehistoric shrimp burrows.
It’s a time-travelling, mind-boggling tour. We attract looks – it transpires that if you peer at something usually considered unremarkable, people stare at you – but frankly, when you’re hurdling geological epochs at every corner, who gives a schist? The Monument itself, of course, commemorates the Great Fire of London, which more than any other event accelerated the use of stone architecture in the capital. The Romans were the first to import stone building blocks here, but it wasn’t until the restructuring of London began in the late 1660s that natural, hard-wearing materials became more commonplace.
There’s nothing commonplace, however, about many of the stones we stop at. Near St Paul’s Cathedral – the steps of which hold 30cm-long fossilised orthocones (“They looked a bit like swimming carrots,” says Ruth) – the limestone exterior of a wine bar displays an even rarer find: a small vertebrate bone from 150m years ago. “Possibly a pterosaur,” she explains, “but we might never know.”
Best of all, perhaps, is the co-working space we pass on Houndsditch, its exterior constructed of gneiss from a meteorite impact crater in South Africa. About 6,000 miles from its place of origin, the stone’s surface is still patterned with crack-like veins of black impact glass, which also contain traces of the meteorite’s extraterrestrial minerals. Oh, and it crashed to earth a mere 2bn years ago. Now there’s something to mull over when January feels like it’s going slowly.
The walk was provided by London Pavement Geology. Ruth leads guided walks with London Walks, £20 for adults on a group tour, walks.com