Several cases of vermiculations formation have been reported in painted caves, with potential issues for the conservation of parietal prehistoric paintings. Vermiculations are natural patterns observed in caves. They result from displacement of sediment initially at rest on cave walls. The collapse of the sediment yield stress, which allows the sediment to flow under small mechanical stresses could be a necessary first step of the vermiculation process. Two possible scenarios have been identified: (1) when the sediment is soaked in low-mineralized water, a rapid and limited drop of the yield stress is followed by a slow decrease, which can seriously weaken the sediment layer if soaking is continued over several months and (2) a spectacular decrease of the yield stress (two orders of magnitude) when the sediment is soaked within a solution enriched in mono-valent cations for weeks and suddenly exposed to low-mineralized water. The specific behaviour of smectite clays binding the sediment accounts for this loss of cohesion .