By digging for just a few hours on either side of low tide, archaeologists have uncovered the world's oldest tidally powered corn mill. Andrew Read reports
Tidal energy is cheap and renewable if you can harness it. Irish monks did - 1,500 years ago. Working at Strangford Lough, in Co Down, archaeologists have found the world's oldest tidal-powered corn mill. And about a million tidal cycles after it last worked, they got part of it going again.
The mill is the only one to reveal the advances made by ancient mill engineers, according to Thomas McErlean, one of those who found it. "It's very exciting. We can trace through the 7th and 8th century increasing sophistication in hydraulics and design."
Inter-tidal archaeology is not easy. The sea covers your dig twice a day, and even when you can work pumps are needed to keep the sea out. But tidal coasts are the last frontier in archaeology, says McErlean. With Rosemary McConkey and Wes Forysthe of the Centre of Maritime Archaeology at the University of Ulster, he discovered the mill during a five-year survey of the lough.
The mill, just below a monastery on Mahee Island, was used to make the monks' daily bread. The first mill, built in the spring of 619, had a 2,000-square-metre millpond. The pond formed when the retreating tide was trapped behind a triangular wall, cutting off a corner of the bay. Once the tide was low enough, water from the pond would be channelled through a wooden penstock - a box with a narrow opening - onto a wheel with boards used as paddles.
Over the next two centuries the mill was moved higher up the shore and improved. The last mill was built in 879. It had a rectangular millpond, 140 metres by 30 metres. "They worked out that this would be more efficient," says McErlean. The wheel also became more sophisticated, with spoon-like paddles, and the upgraded penstock was larger and made of sandstone.
"The stone penstock is a beautiful thing. We actually got it working. It was quite emotional to watch water shooting out of it, perhaps a million tidal cycles after it was last used." The final mill went out of use very rapidly in the middle of the 9th century. "Perhaps there was a big tide or an attack on the monastery."
The mill was discovered as part of a survey of the foreshore of Strangford Lough funded by North Ireland's Environment and Heritage Service. At low tide 50 square kilometres of the lough is dry. McErlean and his colleagues began their survey by line walking. "What we saw at first was very hard to understand. We were on a very steep learning curve. After 10 years in the sea a 10-year old wall looks much like a 2,000-year old wall," he says. And the foreshore is full of walls. During the 18th century, when seaweed was harvested to make soda ash, the area was subdivided to define property rights.
"Our biggest problem in those first years was all those bloody walls. It took us two years to work out what was what. When we'd finished we had some walls left over - and those turned out to be the tide mills," says McErlean.
The walls of the millponds were massive structures, with clay cores built around wooden posts and stones pressed on to the clay. The very precise dating of the mills was possible because the ring patterns of the oak posts could be matched with the 7,500-year record of oak-tree rings developed by members of the palaeoecology centre at Queen's University Belfast.
"Some of the posts had bark with signs of early season growth, so we know it was felled in spring," says McErlean. David Brown of the palaeoecology centre says: "The wood still had bark attached, so it must have been used in the few weeks after it was felled."
"We developed tremendous respect for the people who built the walls," says McErlean. Even working from 4.30 a.m. until well into the evening, they still only got about four and a half hours a day free from the sea. "And we were digging with pumps going all the time," he says. "We had to anchor things in our trenches and we still found things floating away. We're not quite sure how they managed to build those walls."
The archaeologists found many other remains, including 5,000-year-old log boat and numerous fish traps dating from the 7th to the 13th centuries. These were large v-shaped structures pointing to low water. Fish were trapped behind them as the tide went out. The early ones were wooden and 100-200 metres long. "But again we could see lovely innovation," says McElearn. After the Anglo-Normans took over, stone walls up to 400 metes long were used. By the 13th century the area was probably exporting fish to feed armies.
The mill and the team's other discoveries are summarised in their book Strangford Lough: An Archaeological Survey Of The Maritime Cultural Landscape, published by Blackstaff Press. Earlier this year all 1,500 copies sold out in three weeks. A reprint is due shortly. The book has been nominated for this year's British Academy Book Prize, the winner of which will be announced in November.