Chronology of the Late Roman Antiquity walls of Le Mans (France) by OSL, archaeomagnetism and radiocarbon

Résumé En

The Late Roman Antiquity walls of Le Mans in northwest France are one of the most representative and preserved examples of the urban fortifications that developed in the Roman provinces of Gaul and Germany at this period. Because of a lack of reliable chronological data, the construction of the walls was poorly dated, which made unclear its historical context. The main objective of this study was to reassess the date of construction using several methods on a well-preserved sector (sector 11) of the Late Roman Antiquity walls. The dated masonries, thoroughly studied in building archaeology beforehand, are characterized by an alternating of stone and brick courses. Sampling focused on one hand on mortars with four radiocarbon dates (14C) and six single-grain optically stimulated luminescence (SG-OSL) dates, and on the other hand on bricks with two OSL dates (quartz fine grain technique) and one archaeomagnetic date on a set of 104 bricks. The consistency between the dates on the two types of materials discards a possible reuse of the bricks from former Roman buildings. They were produced for the construction of the walls with the presence of several types of bricks likely reflecting a supply from at least two workshops. The dating program in the sector 11 also included 21 14C dates and six SG- OSL on protohistoric structures, Early Empire masonries and large medieval buildings, in order to investigate the evolution of the area over the long-term and to better constrain the chronology of the Late Roman Antiquity walls in Bayesian modelling. The chronological model (Chronomodel software) dates this construction between 301 and 423 CE at 95% of confidence. This date clearly excludes that the Late Roman Antiquity walls were built during the crises of the 3rd century, as previously thought, but rather in the stable political and economic context of the 4th century.