They came to light not through any planned excavation but around 1978, when construction workers extending the Mitsui Denman factory disturbed ground that had been undisturbed for over thirteen centuries.

The two mills, one horizontal-wheeled and one vertical-wheeled, have both been dated to around 630 AD. A horizontal mill, sometimes called a Norse or Celtic mill, uses a wheel laid flat in the water with paddles angled to catch the current, driving the millstone directly above without the need for gearing; the vertical-wheeled type is closer to the familiar image of a mill wheel turning on a vertical axle beside a stream. What makes the horizontal mill here particularly unusual is the character of its penstocks, the wooden channels used to direct water onto the wheel. Two hollow-log examples were recovered, and the researcher Colin Rynne, writing in 1992, noted that they were wholly unlike the normal type: comparatively short at just over two metres each, but with strikingly tall sides of around 81 centimetres. The millhouse sub-structure itself survived as wooden framing for an inlet channel, with side timbers and plank flooring. Immediately to the south, the vertical-wheeled mill left its own footprint: wooden framing for a headrace channel, partially floored with wattle, narrowing toward a two-piece wooden penstock just over three metres long. Together, the two structures represent a rare instance of early medieval industrial archaeology preserved almost by accident in waterlogged estuarine ground, on a tidal island that the modern world has since absorbed entirely into its working infrastructure.