When the tide recedes at this point on the Severn estuary, rare evidence of stone age activity is uncovered. Time Team are on a three-day mission to help recover some of these relics before they are washed away. It involves excavating and painstakingly examining 15 cubic metres of muddy silt; but time is against them. The Mesolithic period is poorly understood, because these people were highly mobile hunter-gatherers who did not build permanent structures.