Archaeologists often grapple with the challenge of piecing together the history of ancient structures from mere ruins. This was the case for the remnants of the Roman water mills in Barbegal, located in southern France and dating back to the 2nd century AD.

This remarkable industrial complex featured 16 water wheels arranged in parallel rows—eight on each side—operating in a cascading system. Known as the most significant Roman hydraulic complex, it represented the highest concentration of mechanical power in the ancient world.

Initially, the sparse ruins revealed little beyond the fact that the wheels were supplied by an aqueduct channeling water from nearby hills. A coin from Emperor Trajan’s reign found in a basin above the mills, along with structural features, indicated that the mills were in operation for approximately 100 years.

Situation of the Barbegal mill complex and the carbonate deposits in situ. (a) Situation of the Arles aqueduct and the Barbegal mills in France. Only a part of the north branch is shown. (b) Aerial view of the Vallon des Arches, with the arrangement of the aqueduct of the city of Arles (aqw), and of the aqueduct that feeds the mills of Barbegal (aqe). (c) Aerial view of the mills of Barbegal, facing east. The red (Group 1) and blue (Group 2) markers in (b, c) indicate where there are carbonate fragments or deposits in situ in the place. (d) Reconstruction of the mill complex. (e) Schematic presentation of two superimposed mill basins with pipelines, as exposed in this work. (f) Cross section of a mill basin with mill wheel and channel.
Situation of the Barbegal mill complex and the carbonate deposits in situ. (a) Situation of the Arles aqueduct and the Barbegal mills in France. Only a part of the north branch is shown. (b) Aerial view of the Vallon des Arches, with the arrangement of the aqueduct of the city of Arles (aqw), and of the aqueduct that feeds the mills of Barbegal (aqe). (c) Aerial view of the mills of Barbegal, facing east. The red (Group 1) and blue (Group 2) markers in (b, c) indicate where there are carbonate fragments or deposits in situ in the place. (d) Reconstruction of the mill complex. (e) Schematic presentation of two superimposed mill basins with pipelines, as exposed in this work. (f) Cross section of a mill basin with mill wheel and channel. Credit: C. W. Passchier et al.

However, the specifics of the mill wheels’ design, their function, and their usage remained elusive until recent research shed light on these mysteries.

A team led by Professor Cees W. Passchier and Dr. Gül Sürmelihindi from Johannes Gutenberg University of Mainz (JGU), in collaboration with French and Austrian colleagues, has uncovered the history of the Barbegal mills through calcium carbonate deposits now housed in the Arles Archaeological Museum.

These deposits formed towards the end of the mills’ operational life, accumulating on the sides and base of the wooden supply system that directed water to the wheels.

We demonstrated that it is possible to largely reconstruct the history of a water mill based on such carbonate deposits, stated Passchier. The researchers first assembled some of the 140 stored pieces like a puzzle and then analyzed the layers using various techniques, including mass spectrometry.

Model of the Barbegal complex, at the Musée de liarles antique
Model of the Barbegal complex, at the Musée de liarles antique. Credit: Carole Raddato / followinghadrian.com / Wikimedia Commons

The findings, published in Geoarchaeology, revealed that the wooden water wheels and channels needed replacement every three to eight years. In one instance, an old water wheel was replaced with a larger one.

This conclusion was drawn from the unusual shape of the carbonate deposits in the water channel. Lower, earlier layers indicated initially low water levels, while upper, later layers suggested higher water levels.

The team dismissed the possibility of an initial low water flow increasing over time. Instead, they concluded that the water channel’s gradient must have changed from steep with a low water level to a gentler slope carrying water at a higher level.

The entire structure of this water mill must have been modified, Passchier explained. When you raise the water channel, you also need a larger water wheel to efficiently drive the system.

Interior of the aqueduct channel
Interior of the aqueduct channel. Credit: Matthias Kabel / Wikimedia Commons

Isotope analysis of the carbonate layers allowed researchers to determine the operational periods before parts of the mill required renovation. By analyzing oxygen isotopes in the carbonate, they inferred water temperatures and identified the seasons during which layers were deposited. They concluded that the carbonate in the Arles samples was deposited over seven to eight years.

The uppermost carbonate layer contains shellfish and wood fragments, indicating that the mill had been abandoned and was disintegrating. Water continued to flow for a while, so the deposits kept forming, but maintenance had ceased, Passchier noted.

Further analysis showed that the mills operated independently towards the end of their life, with the west side being abandoned before the east. Additionally, long pieces of carbonate from the channels were later repurposed as partition screens in a water basin for other industrial uses after the mills were abandoned.

SOURCES

Universität Mainz

Passchier, C. W., Sürmelihindi, G., Viollet, P.-L., Leveau, P., & Spötl, C. (2024). Operation and decline of the Barbegal mill complex, the largest industrial complex of antiquity. Geoarchaeology, 1–15. doi.org/10.1002/gea.22016

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