19 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c001 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE The Poitou Threshold T. Gaillard and P. Branger Location and definition The Poitou region is a natural area located between the Paris and Aquitaine basins, and between the Massif Central and the southeastern terminus of the Armorican Massif (France). This unique geographical configuration has led Poitou to be alternatively conceptualized as a strait (Longuemar, 1870; Welsch, 1892; Fournier, 1888) or as a threshold (Welsch, 1892; Gabilly, 1962). The term “strait” refers to the marine channel that once connected the Paris and Aquitaine basins, while the term “threshold” denotes a region of intermediate elevation located between higher and lower elevations. From a geological perspective, the Poitou region is characterized as a strait, whereas geographers consider it a threshold. Both terms describe the same physical reality. “Geologists refer to this region as the Poitou Strait to indicate its role in connecting the sedimentary formations of the Paris Basin with those of the Aquitaine Basin. This concept of a strait stems from the interpretation that the Jurassic deposits of the Poitou Threshold represent sediments from a marine channel or strait extending between the Paris and Aquitaine basins” (Welsch, 1903). Welsch further elaborates: “Since the region around Poitiers is, in fact, a zone of lower elevation relative to the Limousin and Vendee areas that it separates, it is appropriate to designate it as the Poitou Threshold”. The threshold is thus defined by its intermediate elevation, its position between low-lying sedimentary basins and higher ancient massifs (Fig. 1). This geographical definition was adopted by Welsch, who described Poitou as “a vast plateau 1
RkJQdWJsaXNoZXIy NjA3NzQ=