for National Geographic News
London's oldest known timber structure could be the city's earliest "boardwalk," archaeologists say.
Preserved for more than 5,700 years, the structure was found in an ancient peat bog next to the Belmarsh prison in Plumstead, a suburb of East London near the banks of the River Thames (see map).
"It is definitely man-made, and a very rare find," said team member Jon Sygrave of the Institute of Archaeology at University College London.
At the time the timbers were laid down, the Thames was made up of numerous interweaving tributaries and channels, which flowed through a vast marshland.
The structure was most likely built to keep people's feet dry as they ventured across the soggy ground near the river.
"It probably provided access into a resource-rich area full of birdlife and plants and [was] close to the river for fishing," Sygrave said.
The boardwalk is one of just a handful of similar ancient structures that exist in the U.K.
The oldest known timber trackway in the country is the Sweet Track, which was built about 6,000 years ago across marshes in what is now Somerset.
But the Belmarsh structure is London's oldest, predating by 700 years a timber trackway found in the city's Docklands area in 2002.
The newfound platform is about 5 feet (1.5 meters) by 6.5 feet (2 meters). It was made from split alder or hazel logs that were each about four inches (ten centimeters) wide.
The wood beams were found 15 feet (4.7 meters) underground near the remains of a now dry river channel, the team said.
But it's not clear how far people might have traveled to reach this boardwalk, the excavation team said, since no prehistoric settlements have been found nearby.
Further analysis of the structure as well as ancient preserved plant material found around it should help clarify the trackway's purpose.
Archaeologists found the structure during excavations carried out before construction of a new prison building.
The structure may extend farther into the ancient bog, but the complexity of the excavation meant it would take too much time and money to investigate further.
Instead, any additional timbers will remain buried, and the excavated area will be preserved under special glass flooring in the new building.
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