When the glaciers melted vast amounts of water were released, generally following the route of the streams and rivers that we see today. During this period the sea level was some 200 metres lower than it is today. This large amount of water had a great erosive power, carrying a vast quantity of debris and flowing beneath the glacier under high pressure, to produce tunnel valleys. Since these valleys were confined by the ice this caused the water to pass along deep incised water routes with irregular bases, before rising at the glacier snout to flow from the edge of the ice sheet as outwash or meltwater streams.

The movement of the water followed the lie of the land and the movement of the ice from the north west toward the south east. Undoubtedly the tunnel streams began on the plateau, though there is some evidence that they tended to retreat with the melting ice sheet.

Glacial sands and gravel are present the whole length of the River Stour. Bedrock in Thurlow is some 20 metres below the present river level, the infill being mainly boulder clay with approximately 4 metres of glacial sands and gravel. The same tunnel valley that þowed through Thurlow can be seen at Wixoe (fig. 4), where its lowest point is approximately 40 metres below the present sea level, while at Clare it is as far as 110 metres below our present sea level. Except for the tunnel valleys it is probable that the ice sheet, which produced the chalky boulder clay, rolled upon a bed of glacial sand and gravel, producing the general flattened character of the area which we see today.