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Fig. 13


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A. Following the incision of the Vis River canyon, the hydraulic gradient and associated karstic system are flowing northward. The karstic reservoir consists of a network of ‘dissolution-collapse breccia corridors’ oriented N-S and ghost-rock associated with their residual alterite. A soft sedimentary cover lying unconformable over the carbonate allows for alluvial dynamics. B. The pocket valley initiates to the west from the Lergue River, resulting in a drop of the water table and the emergence of a karstic spring at the intersection between a corridor and the scarp. The residual alterite is rapidly evacuated and the corridor centre collapses creating caves large enough to be explored by humans. The soft covering formation is evacuated by withdrawal through the karstic system. C. Regressive erosion progresses eastward, as well as the depletion of the water table. New corridors become exposed to gravitational effects and collapse as the residual alterite is evacuated. Each karstic spring defines a pocket valley head, which propagates northwards, guided by corridors. D. In a later stage, E-W tube galleries connect isolated ghost-rock features to the corridor network. Residual alterites are evacuated by flooding/dewatering of galleries even beneath the overflow spring position during high-flow events. The combined effect of the absence of a marly seal unit and concentrated infiltration due to the St. Maurice polje allows for the development of epiphreatic speleogenesis. Regressive erosion, still propagating northward, leads to the capture of the karstic system initially flowing towards the Vis River canyon (i.e., Cochon Cave).

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