London rocks

Our British geological 'hero' William Smith might not recognise many of London's buildings today, but there are still plenty of landmarks that would be familiar to him. He could spot the dome of St Paul's (founded in 604, finally rebuilt by Sir Christopher Wren over a period of 35 years from 1675), the White Tower of London (built in 1097) and the remnants of London's Roman Wall (built around 200 AD). And Smith could still find his way from the Tower to St Paul's because so many of the street names are the same as when he lived in London in the early decades of the nineteenth century.

London, like many great cities of the world such as New York, Paris, Moscow or Beijing, can be seen and thought of as a very particular kind of geological phenomenon - environments built of enduring rock materials that stand a good chance of being 'recruited' in some form into the future geological record. Just as archaeologists today grab any opportunity to probe the deep foundations of modern cities, as they try to unravel the early development and history of settlement, future archaeologists will be trying to recover information about what happened in the first decades of the twenty-first century.

Already, so much of the London environment that was familiar to William Smith has gone, mostly destroyed by the Luftwaffe but also greatly helped by really uninspired and ugly post-war 'redevelopment'. Some idea of late Georgian London can be recovered from historical documents, but by no means all. Examination of the foundations of today's new buildings often exposes artefacts and other material evidence of older buildings and activities. Furthermore, the construction of high-rise modern buildings has required deeper and more substantial foundations than previously used. The excavation of these deep foundations penetrates far below merely historic ground levels into prehistoric Pleistocene or earlier times. In doing so, it has revealed some fascinating insights into Ice Age environments, a few hundred thousand years ago (see p. 53).

The growth and development of human settlements are just like many natural geological processes. Construction and destruction combine to leave a cumulative record of layers piled one upon another, although the layers mostly consist of debris and the footings of successive edifices that were originally much grander. Both the city archaeologist and the geologist are frequently confronted with fragmentary 'palimpsest-like' records from which they struggle to reconstruct that past.

I know that many people have difficulty seeing what archaeologists or geologists find so fascinating about a muddy building site, quarry, roadcut or cliff face. Why do they spend as much time as possible grubbing about nose to the ground, backside in the air, in all weathers, scraping away with small trowels and hammers and peering at miniscule details through magnifying glasses? Well, they are like detectives or forensic scientists at the scene of a crime, often racing against time or developers to recover that elusive story of a lost past and trying to reconstruct the scene as best they can. It can be a surprisingly seductive and addictive pastime and one that is not just confined to the common caricature of bearded archaeologists dressed in shorts and sandals no matter what the weather.

Being a civil and mining engineer, William Smith had a unique knowledge of stone quarries around the country and would recognise many of the rock materials used for London's buildings. The construction of stone buildings in London has always been a problem because of the lack of any hard rock in the vicinity, as the Romans discovered. Growth and changing function required that at least some buildings, especially any fortifications, be made of tougher stuff that could not so easily be destroyed by fire. All of London's stone has been imported from elsewhere in the country and nowadays from all over the world. Being on a navigable river allowed building stone to be brought in quite easily. Certainly the Romans were past masters in the business of heavy seagoing transport, masonry and engineering construction.

When Julius Caesar's legions overran Gaul between BC 58 and 51, they found a region rich in rock suitable for building, but with no vernacular tradition of domestic stone working. However, when the Roman Emperor Claudius conquered Britain nearly 100 years later in AD 43, he found that the southern part of the country did not have much in the way of readily available rock for building. The Romans must have been taken aback by the lack of stone, but they were great improvisors and apparently good geologists because they soon 'sniffed out' what building stone was available. They found a well-established trading centre on the banks of the Thames, which they took by surprise and easily subdued. Roman London was firmly established (between 43 and 50 AD) and they built the first bridge across the river out of wood.

The Roman habit of enslaving the locals did not go down too well and in AD 60, a local heroine by the name of Boudicca and her wild army of Iceni and Trinovantes from Norfolk wreaked revenge on the Roman settlement of Colchester, routed Emperor Nero's 9th legion and then burned London to the ground. Archaeological traces of the fire can still be found in a layer of charcoal and red oxide from incinerated mud and wood. Needless to say the Romans, led by Suetonius Paulinus, were soon back in force. This time they set up shop with their standard city kit, including a public forum, basilica, temple, amphitheatre, baths, shops and markets, along with a fort. Finally in AD 200 they built a great 6 m (20 ft) high surrounding wall of stone about 5 km (3 miles) long and reinforced the river embankment to protect the 30,000 or so inhabitants.

The one local rock-derived material that the Romans did use was clay to make bricks, again a technology at which they were expert. But the stone for the fort and wall had to be shipped in by river from quarries near Maidstone, 70 miles downstream in Kent. The rock is known as Kentish Ragstone and was to become one of the most important stones for building purposes in southeast England. It is a grey limestone of early Cretaceous age (some 116 ma old) and although hard is not very good for carving or ornamental work.

Elsewhere in Britain the Romans used whatever stone was locally available and they found excellent freestone in the southwest, such as around Bath. Good-quality material they were prepared to move over considerable distances by road and water. A large ceremonial arch over 25 m high at Richborough in Kent was built of Ragstone and then clad in 400 tonnes of Carrara marble imported from Italy. And even the much smaller London ceremonial arch required the transport by road of over 100 waggon-loads of limestone from Lincolnshire 145 km (90 miles) away. Altogether London's Roman remains show that over 20 different ornamental stones, mostly marble, were imported from France, Italy, Greece, Turkey and Egypt, although the total bulk would not have been great.

The Kentish Ragstone was also exploited by the Normans some 800 years later when the next phase of significant building took place in London. The Ragstone was used to construct the impregnable walls of the White Tower, now part of the larger Tower of London. However, when it came to important ceremonial buildings such as Westminster Abbey, the Norman masons imported from France the rock material they were most familiar with, the beautiful white Caen limestone.

Smith would have recognised the Ragstone, but many of the foreign stones in use today, such as the orbicular granite from Finland and limestones from Turkey, would certainly have puzzled him. Nevertheless, the fabric of the city today, just like that of most modern human settlements, is still largely constructed from natural earth materials. Whether its modern glass, steel and concrete or the more traditional stone, brick and slate, they are essentially all derived from rocks. At any moment of the day or night, somewhere there are vast quantities of rock and mineral material being mined or quarried, then transported and processed from their raw state and finally fabricated into the buildings, roads, walls, bridges and so on that we take for granted.

These days the proportion of rock used straightforwardly as building or dimension stone, as it is called, is very small because it is so expensive. Less than 0.25 per cent of all geological materials used in construction is dimension stone (even so, around a million tonnes of dressed stone is used each year in the UK). The vast majority is crushed into rock aggregate for road construction and concrete along with sand and gravel, in total some 250 million tonnes a year in the UK. Most of us never really witness the extraction end of the business because it is unsightly, noisy, dirty and dangerous and we do not like it in our own backyards, but it still happens - out of sight, out of mind. Worldwide we are talking about over 11 billion tonnes of solid rock being quarried for construction, along with another 9 billion tonnes of sand and gravel, not to mention all the rock material excavated by the mining industry.

Million Years Ago HOLOCENE

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