The best view of a newly discovered archaeological site in Kent is from the trains thundering past a few feet away. Passengers heading towards the Ramsgate ferry ports glance incuriously out at what was a jungle of brambles and nettles a few weeks ago, not realising that they are seeing almost 2,000 years of history rewritten.

The recently uncovered structure at Richborough is a small medieval dock, neatly constructed by joining up double-decker-bus-sized lumps of Roman walls which tumbled and slid down from the ramparts of the fort further up the slope. It is built on the shingled Roman shore, one of the key sites in the Roman invasion of Britain in 43AD, and can be securely dated to the 14th century, since the construction technique is identical to the medieval town walls of nearby Sandwich.

The problem with the discovery is that according to the conventional history of the site, Richborough had been completely filled with silt 800 years years earlier, the once magnificent Roman fort and large town left abandoned and desolate.

The little dock, still filling with water seeping from under the railway line, proves that at the height of medieval Sandwich's power and wealth as a port - the town is now as landlocked as the fort - boats were still mooring at Richborough.

"This really leaves us with a lot of questions," English Heritage archaeologist Tony Wilmott said, scratching his head, "I'm going to have to go away and spend the winter thinking hard."

Richborough Roman fort now stands among farm fields and scrap metal yards, in the shadow of power station cooling towers, on a windy ridge two miles from the sea. Its sea channel and dock gave shelter from the shifting sands and silty water off Ramsgate, infamous among sailors throughout history. Thousands of shipwrecks still lie buried in the mud.

The fort was once one of the most imposing Roman sites in Britain, and despite being used as a convenient builder's suppliers for cut stone for centuries, its towering broken walls and huge earth banks are still commanding.

Finds from the new excavation include fragments of white marble from the huge triumphal arch built to mark the conquest of Britain. Most of this was later stripped and ground down to make limestone mortar for an Anglo Saxon shoreline fort; nothing remains of the arch except the foundations.

The amphitheatre and town still lie buried under green fields and, as the coastal edge of the site eroded, massive sections of the outer wall collapsed and tumbled down the slope. Some landed upside down, chalk foundations in the air, still held together by the strength of the Roman cement.

"It must have been absolutely terrifying when it happened, like an avalanche - I imagine some poor bugger down by the shore looking up and saying, 'what's that noise?' …" Wilmott said.

What Wilmott was actually searching for was the Roman dock and the grand landing stage and steps which were believed to have led up to the triumphal arch: he has found the Roman beach for the first time, but the dock may still lie under the jumble of fallen walls and the railway line itself.