Some portions of fossil caves show asymmetrical gallery sections and these morphologies can be associated withbiocorrosion. When the highest point of the gallery is not situated in the middle axis, we can observe, just below, an inclinedto subvertical flat surface where the rock is strongly weathered and cut out by karrens and pots. At the foot of the oppositewall of the gallery, a well‐marked basal notch is common. Above the notch, a succession of hollow shapes (conches)intersecting both limestone and calcite is present. The above features, as well as the presence of hydroxyapatite crusts,indicate significant occupation by bats and represent morphologies formed through ancient biocorrosive processes. Theinclined wall, situated under the living areas of the bats, is transformed into a “biogenic lapiaz” formed under a layer of guano.The notches located at the base of the opposite wall correspond to the distal end of the guano heap, the acidity of which,released by mineralization, corrodes laterally. Above the notch, the hollow shapes, which we will call "biogenic conches",result from the upward circulation of gases and acid aerosols generated by the convection resulting from the degradation ofguano. This process of differential alteration tends to increase the initial asymmetry of the gallery whose two walls have avery different morphology and texture.