Abstract Tafoni and honeycombs remain some of the most enigmatic and puzzling geomorphological phenomena. Their globally widespread occurrence across a wide range of lithologies and environmental conditions suggests that their formation is determined by a factor that overarches variations in these conditions and weathering processes. Based on a study of tafoni and honeycombs in the Crimean Piedmont, this paper demonstrates that the primary factor in their formation is the pre-exposure alteration of rocks along fractures and karst conduits as a result of fluid–rock interactions. Such alteration is commonly induced by ascending flow and is related to hypogene karstification. The morphological expression of cavernous features through removal of the alterite can occur under subsurface conditions, but is more frequent on exposure to atmospheric weathering. The local or regional characteristics of a weathering system are irrelevant or of only secondary importance in determining the localization and morphology of cavernous features. New features do not form on rock surfaces where the alteration zone was totally denuded or never present. The proposed model resolves major issues inherent to previous interpretations of cavernous features. It has general applicability and important implications for geodynamic and palaeohydrogeological reconstructions. Typical tafoni and honeycombs may indicate past events of ascending flow and the potential presence of hypogene karst systems.

Abstract Ashalim maze cave, and neighbouring caves in the NW Negev Desert, Israel demonstrate hypogene karst features. These features are shown to have developed as a result of the mixing of two types of groundwater flowing in opposite directions within two tiers of Cretaceous rock aquifers. The stable isotope composition indicates that the lower Kurnub sandstone aquifer was recharged over far-field Nubian Sandstone outcrops in the vicinity of the Precambrian basement outcrops of the Sinai Desert, which belongs to the Afro-Arabian dome. The water flows northward and rises into the Judea carbonate aquifer through deep faults. A similar hydrogeological system is inferred for the speleogenetic period of Ashalim Cave. Dewatering of the cave occurred in the Pliocene due to regional uplift. This is indicated by the first vadose speleothems, dated to the late Pliocene (3.1 Ma). This was followed by surface denudation, which breached the cave and formed the present entrance.