A. In the Hettangian dolomite, a steep-sided thalweg with a flat bottom aligns with the Soubès Cave (location shown in Figure 5). B. Cross-section of the thalweg shows intense vertical jointing. The flat bottom is made of sub-horizontal bedding, in a ≤ 25 m² area. C. The rose diagram displays the orientation data of 26 fracture planes (ranging from 0,1 to 1m-scale) measured at the lower end of the thalweg. D. Within a protected elongated hollow structure (location shown in B), blocks of competent dolomite are embedded in soft dolomitic sand, preserving the initial rock bedding. The pocket knife thrust into the soft dolomite illustrates the in-situ alteration process of ghost-rock karstification.