SEDIMENTARY PROCESSES/Karst and Palaeokarst 685 Figure 10 Relict paragenetic canyon, with sediment removed, extending upwards from the bedding plane on which the anasto mosing ‘proto caves’ first developed The Burren, Ireland cavities are water filled, and water flow follows the path of least resistance, often flowing upwards if necessary Dissolution here occurs in all directions, forming passages with circular or elliptical crosssections, as seen in the drained tubes of Kentucky’s Mammoth Cave and still forming in the active phreas of Mexico’s Yucatan, often with blind pockets formed by eddies or by mixing corrosion where percolation water emerges from a minor fracture into a flooded passage Mixing of karst waters from separate sources may enhance dissolution locally, influencing both conduit configuration and shape Sediment aggradation is common both in vadose and in phreatic passages and may armour the floor against downward dissolution, causing lateral or upward dissolution by paragenesis (this is a process quite distinct from mineral paragenesis) In vadose passages, lateral dissolution may form paragenetic notches at stream level, spectacularly seen in some of the huge river caves of Mulu, Sarawak In phreatic environments, dissolution may raise the roof, forming a largely sediment-filled paragenetic canyon Paragenetic canyons (Figure 10) extend upwards from their initiation level, such as a bedding plane, whereas vadose canyons (Figure 11) cut downwards from it A drop in aquifer base level may flush out sediment fill from paragenetic canyons and/or form ‘keyhole’ passages by vadose incision in the floor of former phreatic tubes The two types of canyon passages can be distinguished by the presence of either undulating wall notches, indicating phreatic paragenesis associated with undulating sediment banks, or horizontal notches, indicating vadose conditions Keyhole passages, with a vadose trench incised in the floor of a phreatic tube, are particularly significant for interpreting the history of karst Figure 11 Active vadose canyon incised below the inception level at which the cascade is entering Doolin River Cave, The Burren, Ireland aquifers, indicating rejuvenation when base level drops below the cave passage Water in a karst aquifer may be confined by an impermeable rock (aquifuge), whereas impermeable or weakly permeable layers (aquicludes or aquitards) may influence water movement through the rock, allowing the development of perched aquifers above regional base level Hence the altitude of a particular phreatic passage may reflect only local conditions Eddies caused by turbulent water flowing across limestone in caves and surface streams commonly form asymmetric concavities, called scallops, which are invaluable flow indicators in relict cave passages (Figure 12) Their steeper slopes face downstream, and their size is a function of flow velocity Smaller scallops indicate faster flow than larger ones; vadose scallops are sometimes only 1–2 cm across, whereas phreatic scallops may be m across! An approximate rule of thumb for calculating flow velocities (in metres per second) is to divide 3.5 by the mean length of the scallops in centimetres