ENGINEERS working on the route of HS2, the new high speed rail link, have discovered an ancient, sub-tropical coastline dating back 56 million years.

The rare discovery was made at a site in Ruislip, where HS2’s ground investigation team unearthed a previously unknown material, located up to 33 metres below the surface.

The layer of black clay, which HS2 Ltd has named the Ruislip Bed, is thought to have been formed from densely-wooded marshes on the edge of a sub-tropical sea.

HS2 made the surprising find while investigating ground in the area, prior to the construction of the Northolt Tunnel, a 14km section which will run from West Ruislip to Old Oak Common.

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Jacqueline Skipper, a geological expert from Geotechnical Consulting Group, said: “Although ground investigations regularly take place across the country, it’s really exciting and very unusual to come across a material that no-one has ever seen before.

“The Ruislip Bed discovery is particularly fascinating as it is a window into our geological history.

“It would have been formed during the Paleocene period, which was a time of intense change, with new animals evolving following the extinction of the dinosaurs.

“Most of Southern England was covered by a warm sea and this clay helps us to pinpoint where the coastline was.”