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Tunnels like this one beneath London's Royal Docks have been yielding fossil treasure during construction. Image: Crossrail Project, Twitter

Seventy thousand years ago, the area around London’s Thames River was covered with many smaller criss-crossing rivers and streams. These waterways have long since been built over or converted into sewers – but the outlines of some still lie deep under the city. It was one of these hidden tunnels that yielded some ancient loot back in 2011: around 4,000 prehistoric mammal fossils. 

The fossil find was unearthed on the sidelines of Europe’s biggest single construction project: a £16 billion (around $27bn) rail network known as Crossrail, which will create a new underground rail link beneath the British capital. Since 2009, engineers have been building new stations and digging two giant tunnels under the city – and a team of archaeologists has been working right beside them. By law, Crossrail must carry out archaeological investigations alongside its structural works. 

While digs in rural areas allow archaeologists to use the latest technology – like UAVs, satellites and infrared and thermal cameras – to help them uncover hidden artefacts and fossils in the rock and soil, working in a big city is very different. Much of this technology doesn’t work too well in an urban setting, where demolition debris and underground pipes and wires tend to get in the way. That’s why the Crossrail excavations have been carried out the old-fashioned way: with spades, picks and ordinary construction equipment.

A bit of technology that has come in handy is specialised computer modelling software that gives the archaeologists a clearer picture of where significant deeply buried finds are likely to be found. “Archaeologists have to plan their investigation meticulously, and know where areas of interest are, before they start digging,” says Crossrail’s lead archaeologist Jay Carver. That means a lot of research, which is thankfully not as time-consuming as it once was. “So much of the information we need used to be locked away in dusty records vaults in libraries and universities, but now it’s easily accessible online,” Carver explains.

“Over the last four years, Crossrail archaeologists have unearthed artefacts that provide fascinating insights into London’s ancient history.”

Over the last four years, Crossrail archaeologists have unearthed artefacts that provide fascinating insights into London’s ancient history, stretching as far back as Roman times. But they’ve also found evidence of what this part of the world was like long before human settlement.

The fossil loot dug up in 2011, near London’s Royal Oak tube station, was a treasure trove of bones belonging to reindeer and steppe bison. “So many bones in one place shows that large herds of these animals were present in the London area back then,” says Carver.

Two metres tall and weighing in at up to 900kg, steppe bison roamed the Eurasian and North American plains from about 500,000 years ago. Carver explains that analysts from London’s Natural History Museum used a method known as optically stimulated luminescence to work out that the Crossrail bones were 68,000 years old. “This technique can give dates by showing how [much time has passed since] the quartz in the soil around the fossil was last exposed to light,” he says.

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Some of the fossil finds unearthed by Crossrail archaeologists at the Royal Oak site. Image: Crossrail

“Back then, the south of England would have been a cold, tundra-like landscape,” Carver adds. During the ice ages, the climate would have gradually changed from cold to warm and then back to cold again, he explains. “The fossils are from one of the cold stages. London would have been on the edge of the ice pack, so there’d be similar animals to those you’d find in modern-day Siberia.” Adapted to the cold climate, the animals would have migrated during the warmer periods, moving across the land bridge that once connected the British Isles to mainland Europe and then heading northwards.

While the archaeologists have unearthed plenty of human relics at other Crossrail dig sites, they found no sign of them here. “We didn’t find any evidence of human activity in the Royal Oak site,” Carver says. “Lots of the bones had marks on them and we thought at first that this might be from hunting, or where people had used tools to work the bones.”

That turned out not to be the case. When the fossils were sent to the Natural History Museum, scientists discovered that carnivores had made the marks. “Most likely wolves and bears, possibly cave lions,” Carver says. It’s likely that the bison and reindeer had drowned after being washed away in floods and local predators had scavenged the carcasses.

By around 10,000 to 8,000BC, the bison and reindeer were gone from Britain. Carver explains that by this time the area’s climate had grown too hot for them. In addition, the North Sea land bridge became submerged by rising sea levels around 8,500 years ago at the end of the last Ice Age, putting an end to the animals’ back-and-forth journeys across the water.  

The Crossrail project is certainly not the only one to yield buried treasure. Significant archaeological finds have been popping up at construction sites all over the globe, like the two-million-year-old sea lion skull unearthed in Los Angeles, the 50-million-year-old crocodile remains in Brisbane or the possible hadrosaur tail found during the building of an oil pipeline in Alberta, Canada.

Back in the UK, Crossrail’s archaeologists are excited about the remarkable finds they’re brining to light. With forty sites stretching across London, they’re part of the UK’s biggest-ever urban archaeology project. “We’re looking at a complete cross-section of London’s history,” Carver says. 

Top header image: Crossrail