If, on her wedding day, Lady Diana Spencer had glanced down at the chequered paving half-way up the steps of St Paul's Cathedral, while courtiers adjusted her train, she'd have seen delicate white fossils etched into the dark blue limestone - cephalopods from the time of the first fishes, 500 million years ago.
She didn't, probably. Most people find geology a bit of a struggle. Which is a pity, particularly in the capital, because London's architects have gone to immense trouble to embellish their buildings with rare stone from all over the world, and have polished it up so that it is far easier to see on the side of a shoe shop or a church than it ever was in its native habitat. A leisurely walk around St Paul's and its precinct, taking about an hour, offers a window into the world of rocks and the way it intersects with history.
The York slabs that pave St Paul's Churchyard, for example, the familiar paving stones of London, tell a story. In the Industrial Revolution this stone was available at little cost, as it divided off the coal measures. This is sandy limestone from the Carboniferous era. Its advantage is that the limestone is softer than the sand, so as the paving weathers, the sand grains stand up against the surrounding rock and it stays non-slip. St Paul's itself rises out of this 340-million-year-old sea like a white ship, built of stone a mere 140 million years old, from the time of the dinosaurs. To rebuild it and the other City churches - which before the Great Fire were built of poor-quality Kentish ragstone - Christopher Wren chose yellow limestone from Portland in Dorset, which weathers to a creamy white. It was formed from millions of tiny organisms when what is now Britain lay at the bottom of a warm, shallow sea near the Equator. Wren chose what he considered the best quality - fine-grained stone from the so-called Whit bed. It wasn't until the Sixties that Portland's "roach" beds, rough- textured because they are full of shelly fossils, became fashionable. This stone was used for the former St Paul's Choir School on the east side of the cathedral, and in the upper part of the octagonal Bank of Boston House, just next to St Paul's Underground station. Of course, when the cathedral was built, around 1700, all the stone had to be brought up by barge. But by the time the Victorian architect FC Penrose was commissioned to redo the front of St Paul's, he had the benefit of railway transport. No expense was spared. He renewed the tumbledown churchyard with a wealth of pretty rock types from far and wide, and replaced the original, beautiful marble statue of Queen Anne (the reigning monarch in Wren's time), which had become badly weathered by the sulphurous smogs of the Industrial Revolution. The present statue is made of white marble from Carrara, in Italy. Some of the rocks seen here are as distinctive as samples of expensive fabric. Surrounding the statue, for instance, are radially arranged setts of limestone. Here and there, pieces of white marble and black slate have been patched in, but if you ignore these and look at the original Purbeck limestone you can sort it into two types. Some contain shells, and take a polish; the others are rough and full of cavities, just like the cliffs of Swanage in Dorset. The highly polished bollards that stop cars from invading the west front of the cathedral are made from a completely different stone. This is granite from Shap Fell in Cumbria, with a texture like rusty- red tweed with strawberry- sized blotches and smaller, white and black spots. It is igneous rock - formed by fire, but not in a volcano as such. We now know that a huge bubble of hot, molten rock rose very slowly to the surface to form Shap Fell, cooling as it went. The size of the crystals shows that the process must have taken many millions of years; molten rock that spews suddenly out of a volcano has very tiny crystals. …