174 A new concept of karst development based on hydrogeology and geophysics which his model does not adequately explain. Welsch’s model envisions dry valleys as preferential drainage axes for groundwater flowing toward springs located at the contact with Toarcian marls or with alluvial deposits (Welsch, 1912). This model, therefore, implies that springs should emerge at the outlets of dry valleys; however, in practice, springs are also found outside valley axes. Another feature is that the Poitou Threshold is not particularly favourable to the karstification of Jurassic limestones. Hydraulic gradients are low due to the relatively low elevation of the plateaus, around 130 to 150 meters above sea level (m asl) around Poitiers, with riverbeds around 70 to 60 m asl. The plateaus are overlain either by red clays resulting from limestone weathering or by Plio-Pleistocene clayey-sandy sediments that inhibit infiltration. Poor descriptions of drilling cuttings and the lack of geophysical logging have further hindered understanding of the positioning of karst conduits. When a borehole encountered a sudden inflow of water, it was often attributed by Figure 1 Historical conceptual models of groundwater flow on the Poitou Threshold; (a) Longuemar‘s model (1856), (b) Longuemar‘s model modified by Welsch (1912).
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