Travel Fossils: When dinosaurs ruled the City

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. Here and there in the bollards (look at the second small one away from the larger ones, on the north side) you can see a much larger, black splodge. This is a piece of the surrounding rock that broke off as the granite slowly welled up, and fell into the melt and half-dissolved.

The bollards are set on a base of faded navy-blue: another igneous rock, diorite from Guernsey. Beyond this is pale grey granite from Cornwall, probably from Bodmin Moor. These three rocks - Shap granite, Guernsey diorite and swirly Cornish granite - are the ones traditionally used for London's kerbstones. Cheap, man-made concrete has only recently begun to replace them.

The dilapidated Juxon House, on the north side of St Paul's Churchyard, was built around 1960. The stone you see decorating it has been used as cladding on a steel-framed building. Between the narrow strips of Portland Whit bed limestone are tile-like pieces of blue-grey slate. This was originally mud, but has been subjected over aeons to heat and pressure that have hardened it into slate. It is from the Silurian age, and comes from the mountains of the Lake District. Juxon House also has pale grey granite slabs at first-floor level. The black stone level with the pavement is immensely old gabbro from South Africa, formed in the earliest days of life on Earth.

If you continue the walk clockwise around the cathedral, you see on your left, past the red-brick Chapter House, Paternoster House. This, again, is mainly clad in Portland stone, with some panels of pale grey Cornish granite, but the pillars running up from the pavement to the first floor are clad in dark green Carboniferous limestone. This rock contains not only fossils of corals, brachiopods and sea lilies, but also the smudgy marks of burrowing animals such as worms.

One of the most spectacular rock types in the precinct decorates the humble kiosk of the Tube station. This streaky black-and-white stone has been violently squashed and heated to force it to recrystallise and create its wavering patterns. Before its traumatic transformation, it would have looked similar to the diorite seen earlier. The rock comes from the Tatra mountains of the Czech Republic.

Make a short detour across Cheapside to look at the church of St Vedast- alias Foster. You can see Wren's work in the grand facade of Portland stone, but on the right the original mixture of Roman brick, Kentish rag and greensand limestone is visible.

It is also worth crossing New Change on the east of the cathedral to get a close look at the extension of the Bank of England. This curving building has a lower course of blue-grey Scottish granite, below rubbed red brick and Portland stone. But around the doors is travertine, an attractive, streaky, creamy stone from Tivoli near Rome. It is a limestone precipitated from warm water, and is naturally full of cavities; here these have been "stopped" with cement. This stone is often seen as walls and paving in Tube stations.

Bracken House, once the headquarters of the Financial Times, is faced with pink sandstone up to about a metre from the pavement. This rock comes from Staffordshire, and was formed from wind-blown dunes in the Triassic era. If you look closely you can see delicate layering that shows the direction of the wind at that time.

Continuing west, you come to Scandinavian House which, appropriately, is faced with a spectacularly pretty Swedish rock. This is Larvikite, a dark-blue igneous stone with flashing, peacock-blue crystals, from Larvik, near Oslo fjord.

Across the road from the south transept of St Paul's is Peter's Hill House, on which a most unusual rock has been used. The dark blue stone facing its pillars comes from the high Andes of Argentina. It looks rather like granite but is in fact a metamorphic rock, and has white blotches - the mineral cordierite - that make it look a bit like a tapioca milk pudding.

When you get to Condor House, which curves around the south-west end of St Paul's churchyard, you will probably realise that the two types of tweedy building stone used in the cladding are both granite. They both come from Scotland. The grey one is Rubislaw granite, and the pink one comes from Peterhead. Now comes a good test of geological nous. Which of these two granites has the best "xenoliths" - bits of surrounding rock which fell off into the hot granite and gradually dissolved into it as it slowly rose to the surface? You should be able to see, in one of these granites, a variety of grey, coin-sized blotches - some of which are quite dark and uniform (they fell in quite late on, and were hardly changed at all) and lighter, speckled pieces which fell in while the melt was still very hot and went some way towards dissolving into the body of the granite.

That's the end of the walk. But if you aren't fed up with geology, a stroll down Ludgate Hill offers a chance to identify many more examples of beautiful building stone. In fact, for a city built on boring, vomit- coloured clay, London can boast a startling range of interesting rocks.

For more information, see 'Illustrated Geological Walks: London', by Eric Robinson (Scottish Academic Press). The Geology Museum (0171-938 8779) leads geology walks, including one around St Paul's Cathedral on 28 July. The charge for each walk is pounds 5