Education / Out in the field

Earth Science Week geowalks: London’s hidden history

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Imperial College’s Matt Loader takes us on a tour of Piccadilly’s geological highlights…

“I’ve been walking about London for the last thirty years, and I find something fresh in it every day” – Walter Besant, novelist. 1901.

Matt Loader explaining the geology of London to a group of intrepid geowalkers.

Matt Loader explaining the geology of London to a group of intrepid geowalkers.

When I moved to London just over two years ago, I was immediately struck by the history of the place. So much of this nation’s past, and indeed the world’s, has revolved around the famous buildings and the well-known streets of the capital. In many ways, a stroll through London can feel like a walk through history, with modern glass buildings vying for attention alongside thousand-year-old stone monuments.

For a geologist, however, the history of the world is not measured in hundreds or thousands of years, but in millions and billions. The complexities of human civilisation are but a brief, final sentence in a book whose innumerable pages detail truly monumental events; the rise and fall of great continents, the advance and retreat of colossal ice sheets, the battleground of evolution. Geology is the language of this book. By learning to read the rocks, you begin to understand the deep history of our planet.

Straight shelled Ordovician Nautiloids on the steps of St Paul's Cathedral in London

Straight shelled Ordovician Nautiloids on the steps of St Paul’s Cathedral in London

When geologists go out into the field to study the Earth, it can often involve days of travelling to difficult places – not a luxury available to most of us. Luckily, the builders of London and other great cities were not only great architects but keen geologists, selecting the best and most beautiful stones from Britain and beyond to make their buildings. These stones span the entirety of geological time and process, to the point where it is possible to travel backwards into deep time, and around the whole planet, with little more than an Oyster card.

graniteWe begin our journey through time on Piccadilly at the Geological Society, Burlington House, and walk towards Hyde Park Corner.  On the north east corner of Piccadilly and Old Bond Street is a jewellery shop. Aside from the wealth of geological minerals and precious metals on display inside the shop, the columns between the windows on the outside are made from interesting stone. The rock is a polished, red-brown colour, containing some large, flesh-coloured, rectangular minerals. This rock is a ~390 million year old granite from the north of England. All granites form by the slow cooling of liquid rock (magma) deep beneath the Earth’s surface, and melting solid rock requires some special geological conditions.

Around the time of our granite’s formation, the British Isles were divided by a great ocean, with Scotland and the north of Ireland on one side, and England, Wales, and southern Ireland on the other. Plate tectonics caused these two separate continents to come colliding together, uniting the British Isles, and forming a huge Himalayan-style mountain chain in the process.

Deep inside this collision zone, the heat was great enough to melt the continental rocks and form new magma, which cooled to form the Caledonian granites. 400 million years of erosion have exposed the deep roots of this ancient mountain chain to the surface. The formation of the huge pink feldspar crystals, or ‘megacrysts’, which are scattered around the rock, hints at the complex processes involved in granite formation. Geologists believe that these crystals form by the repeated heating and cooling of the granite before it completely solidified, generating a coarsening effect where the smaller crystals get smaller, and the bigger crystals grow larger.

Outside Green Park station.

Outside Green Park station.

Further along Piccadilly, next to the Ritz hotel, the newly constructed entrance to Green Park tube station displays some spectacular rocks.  A pale cream stone is riddled with small spiral-shaped holes and curving white impressions. The detailed intricacy of the stone has led some to believe the look is entirely artificial, when in fact these natural features gives us important clues about an ancient sea shore. This is the 150 million year old Jurassic Portland Limestone of Dorset. The holes in these rocks are the remnants of fossilised sea creatures, whose shells have dissolved away over geological time to leave only the abstract impression of their existence. The sea in which they lived has also long since vanished, swallowed up by the constant recycling processes of plate tectonics. Portland Stone is a very common building material in London, with some of the less fossiliferous layers being quarried to build Buckingham Palace, St Paul’s Cathedral, and the Cenotaph, to name only a few.

100 Piccadilly.

100 Piccadilly.

Rocks of a similar age and environment are observed across the road at 100 Piccadilly. If you look closely at this warm, honey-coloured stone, it’s made up of lots of small, spherical grains and occasional shell fragments. This is a sedimentary rock, the Jurassic Bath Stone, named after its town of origin in the west of England. The small, spherical grains that make up Bath Stone are called ‘ooids’, and thus the rock is an ‘oolitic limestone’, which forms by mineral precipitation in the turbulent waters of a shallow tropical beach. We find similar rocks forming today in the warm waters of the Caribbean, so as remarkable as it may seem, the area around modern day Bath would have been more like the Bahamas around 150 million years ago.

2.6 billion year old Australian granite at the Australian War Memorial, Hyde Park Corner.

2.6 billion year old Australian granite at the Australian War Memorial, Hyde Park Corner.

At the far end of Piccadilly, Wellington Arch marks the historical entrance to London from the west. Walking underneath the arch, and heading left to the edge of Hyde Park Corner, we come across the Australian War Memorial. Like the many other Commonwealth memorials in this area, this curving monument is made of rocks from the nation whose countrymen it commemorates. In this case, the monument is constructed of the Verde Laguna granite of Western Australia. Like the Caledonian granites further up Piccadilly, this green stone forming by the slow crystallisation of liquid rock at great depth within the Earth.

The most remarkable thing about this rock is its immense age. At 2.6 billion years old, these rocks are over half the age of the Earth. At the time of their formation the world was a very different place. It was spinning faster, such that there were over 420 days in every year, and each day lasted only 20 hours. Life on the planet was in its most primitive form, only simple cyanobacteria inhabited our planet, and they would be responsible for providing our planet with oxygen by the brand new process of photosynthesis.

Next time you take a walk through the city, remember to look around you for the clues to the world’s hidden history.

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