46 A new concept of karst development based on hydrogeology and geophysics Figure 4 shows the location of the different sections of cliff observed. The oldest formations are observed at points known as “Yews n° 1 and n° 2.” At point n° 1, the presence of Graphoceras concavum identifies the Upper Aalenian. This ammonite is located immediately below a bank of white oolites, which is itself topped by banks of bivalves (Fig. 5). These two banks are thought to represent the Laeviuscula zone (not observed), with the D7 discontinuity (leveled at an altitude of approximately 78 m) as a marker for the base of the Lower Bajocian. The “Yews n° 3” site is located at a higher altitude (the zone between “Yews 1 and 2” and “Yews 3” is not observable) and allows the identification of the white oolitic Figure 3 Geological map of the site (after Mourier et al., 1986).
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