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Origin and history

The Saint-Médard church of Chalo-Saint-Mars, dedicated to the bishop of Soissons, was erected as a priory in 1149 by Gozlin, bishop of Chartres, for the benefit of the Abbey of Notre-Dame de Josaphat. From this period remain the flat choir, the stump of the bell tower and the main porch, characteristic of late Romanesque art. The name Saint-Mars comes from a deformation of Saint Médard, patron saint of the building, while the commune, renamed Chalo-la-Raison during the Revolution, regained its appellation of origin in 1801.

The nave, rebuilt after the Hundred Years War (late 15th–early 16th century), presents three ships preceded by two porches, one Gothic and the other Renaissance. A carved cap bearing the date of 1547, and a vault key of the southern collateral is marked 1758. The furniture, including 1867 stalls and stained glass windows signed Lorin (1895), reflects subsequent additions. Three historic bells remain: Louise (1658), Louise Marie (1672) and Antoine Cécile (1891), the latter offered by an assistant mayor.

The priory, attested until the Revolution, was a major religious centre. In 1695 Jacques-Philippe de Prunlé, local lord, married Marie de Savoie there. The building, rebuilt until the 19th century, was included in the Historic Monuments in 1926. His adjoining cemetery, shared with Saint-Hilaire, houses the tomb of Professor Jérôme Lejeune, visited by Pope John Paul II in 1997. The site also preserves traces of medieval fiefs like La Fosse or Le Tronchet, once breaking up the territory.

The church illustrates the architectural and political transformations of the region: seigneurial privileges under Philip I, revolutionary reassignments (such as the transfer of the Louise Marie bell in 1793), and later restorations. Its status as a protected monument underscores its heritage role in the Essonne, between medieval heritage and modern adaptations.

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