Layers of carbonate provide insight into the world of the ancient Romans

Archaeologists face a major challenge when they seek to gather information about buildings or facilities that are left in ruins. This was a particular challenge for the remains of the Roman watermills at Barbegal in southern France, which date back to the 2nd century AD.

This unique industrial complex consisted of 16 water wheels arranged in parallel rows, eight on the east and eight on the west, operated in a waterfall-like arrangement. Little could initially be deduced about the location from these now sparse ruins, except that the wheels were fed by an aqueduct that brought water from the surrounding hills.

A coin issued during the reign of Emperor Trajan, discovered in a basin above the mill complex, and the structural features of the site indicate that the mill was in use for about 100 years. However, the type of mill wheel, their function and how they were used have remained a mystery until now.

Carbonate fragments yield remarkable information

Professor Cees W. Passchier and Dr. Gül Sürmelihindi from the Johannes Gutenberg University Mainz (JGU), in collaboration with colleagues from France and Austria, have unravelled the history of the mill complex using calcium carbonate deposits now preserved in the Archaeological Museum of Arles.

These deposits had formed on the sides and base of the wooden feed system that transported the water to the wheels towards the end of the Barbegal watermills’ approximately 100-year operational life.

“We show that it is possible to largely reconstruct the history of a water mill based on such carbonate deposits,” says Passchier, head of the JGU team. First, the researchers had to fit together some of the 140 stored pieces like a jigsaw puzzle, then they analyzed the layers using various techniques, including mass spectrometry.

Wooden water wheels and gutters were replaced

The researchers have now published their results in Geoarchaeology“For example, we were able to demonstrate that wooden water wheels and water channels had to be replaced after three to eight years. In at least one case, an old water wheel was replaced by a larger one,” says Passchier.

The researchers drew this conclusion from the unusual shape of the carbonate deposits that had formed in the water channel. While the lower and earlier layers indicate that the water levels must originally have been relatively low, the upper and later carbonate layers indicate a higher water level.

The possibility that there was originally less water flowing through the water channel and that this subsequently increased was rejected by the researchers. They found that – for a gently sloping water channel and a low water level – the amount of water supplied would not have been sufficient to drive a mill wheel.

Therefore, the slope of the water channel must have changed: from an initially steeper slope with a low water level to a shallower slope that transported water at a correspondingly higher level.

“The whole structure of this watermill has to be modified,” Passchier said. “If you just lift the water channel, the water tends to splash, which loses the ability to drive the wheel efficiently. So if you lift the water channel, you also need a bigger water wheel.”

This conclusion is confirmed by a part of the carbonate deposit that has formed on the water wheel. This deposit does not contain all the carbonate layers, but only those from the last years of operation.

Results of isotope analysis provide evidence of the mill’s longevity

Using isotope analysis of the carbonate layers, the researchers were even able to determine the operational periods before parts of the mill had to be replaced. Carbonate contains oxygen and the relative proportions of oxygen isotopes differ depending on the water temperature. Based on the isotopic composition in the carbonate layers, the researchers were able to deduce the water temperatures and thus identify the seasons in which the layers were deposited.

They concluded that the carbonate from the samples in the Archaeological Museum in Arles had been deposited in the water channels over a period of seven to eight years.

“The upper and therefore youngest carbonate layer contains mollusc shells and wood fragments, which shows that the mill must have been abandoned at that time and was falling apart. The water continued to flow for a while, so that carbonate deposits continued to form, but the maintenance of the water channels stopped,” says Passchier.

The researchers were able to answer another question. It was previously unknown whether the mills were controlled in combination by one operator or whether the 16 water wheels were used independently.

Judging from the layers of three examined water channels, which clearly differ from each other, the mills were in operation separately, at least towards the end of their lifespan. Furthermore, the western side of the complex was abandoned earlier than the eastern side.

Finally, long stretches of carbonate from the water channels were later used as partition walls in a water basin for other industrial purposes, after the mills had already been abandoned.