684 SEDIMENTARY PROCESSES/Karst and Palaeokarst autogenic, where derived from a karst catchment, or allogenic, originating on non-karst rocks Commonly, input to a karst aquifer is mixed Autogenic recharge is diffuse, as rainfall across the entire karst outcrop, whereas allogenic recharge is concentrated at discrete stream sinks or swallets Most negotiable caves are associated with such swallets Water re-emerges from a karst aquifer at a rising (also known as a resurgence or spring) Typically, this occurs at the lowest point that the karst rock is exposed at the surface, although impermeable layers within a karst aquifer may cause a rising to be perched at a higher level Resurgences may be free-draining streams or may rise from depth under hydrostatic pressure The more than 300 mdeep Fontaine de Vaucluse in France is an example of the latter; the term ‘vauclusian spring’ has been coined to refer to the point of emergence of water under pressure Caves (Endokarst) Underground drainage conduits are a fundamental element of karst Those large enough for a person to enter are called caves, an artificial definition constrained by the size of the caver! In fact, water can move slowly along submillimetre fractures, but at widths of $5 mm laminar flow gives way to turbulent flow (the turbulent threshold, or hydraulic jump) and dissolution rates increase significantly Such ‘protocaves’, often anastomosing networks of small tubes, commonly are found on fracture planes adjacent to larger caves that have developed from them The development of a cave, from inception to destruction, is called speleogenesis Fractures commonly represent the initial routes for water flow, and the configuration of many passages is strongly influenced by the spatial distribution and orientation of joints, faults, and bedding planes Rectilinear passage networks arise where inception is dominated by vertical fractures, whereas meandering passages imply inception on subhorizontal ones Passage shape may be influenced by these fractures but also is strongly controlled by passage location with respect to the ‘water table’ Above, in the vadose zone (see Figure 9), water occupies only the lower part of the passage and flows downwards, as in surface streams and waterfalls; dissolution here forms canyonlike passages and vertical shafts Sequences of deep vadose shafts and canyons, sometimes descending a kilometre or more, are characteristic of high mountain karst regions such as in the Alps Below, in the phreatic, or ‘saturated’, zone, fractures and Figure Diagrammatic section through a cave system to show the various features discussed in the text