Excavations carried out by Oxford Wessex Archaeology in the Ebbsfleet Valley in north-west Kent ahead of the construction of High Speed 1 uncovered the remains of an extremely well-preserved tidal mill on the bank of the Ebbsfleet stream at Northfleet. Dendrochronological dating indicated a likely construction date of AD 692, making the mill, with a horizontal waterwheel, the earliest of its kind in the country and an exceptionally important discovery. The study of the preserved timbers has, in particular, added substantially to our knowledge of Anglo-Saxon woodworking techniques in this period.

So how did it work? By capturing water in a pond at high tide, and then releasing it through the mill at low tide, the millers could operate two mill wheels for up to three hours, producing around 30 kg of flour.

The water ran from the pond through two square funnels, or pentroughs, made of hollowed-out tree trunks, and the jets drove two horizontal paddle wheels. Each wheel was connected by a shaft to a pair of millstones on the milling floor above. Once the water had passed the wheel, it ran along the mill tailrace and joined the main stream channel. There was even evidence that the mill had a system of levers to allow the miller – while standing on the milling floor above – to raise the water wheel and shaft and so separate the upper and lower stones while they were turning.

The milled flour was collected, and at high tide, when the waterwheel would be under water and the mill would not be operating, a boat would load the flour from a jetty alongside the tailrace, and head off downstream before the tide ebbed again.

We do not know who built and operated the mill, but the church or a king – or both – are strong possibilities. One of the great early kings of Kent, Wihtred, came to the throne just a year before the mill was built, and Archbishop Berhtwald was appointed just a year later.

The mill may have been almost entirely made of wood, but a mill like this was a very sophisticated machine, and would have been built by a skilled and experienced builder, possibly one brought over especially from the continent. The wood used for the mill was almost all oak, and mostly from quite young trees. Unlike the Romans before them, the Anglo-Saxons did not use saws, and every piece of timber in the mill was shaped with axes. But the builders could be precise when they had to be. To calculate the spacing of the waterwheel blades they drew precise patterns of circles with a compass on one of the planks.

The mill stood on its own in a landscape of open grassland dotted with clumps of trees. Apart from the miller, nobody lived nearby – it was too wet. The sea level was rising at this time and within perhaps only 20 or 30 years, the paddle wheels were no longer clear of the water for any length of time, so the mill could not operate. When the mill was abandoned, all the important mechanical parts, like the millstones, the shaft and the water wheel, were taken, and the rest was quickly covered by silt.