SEDIMENTARY PROCESSES/Karst and Palaeokarst 681 Figure Temperate marine biokarst, encrusted with Mytilus, in the intertidal zone The Burren, Ireland Figure Limestone pedestal beneath a sandstone erratic The pedestal provides a measure of postglacial ($14 ka) surface lowering of the surrounding limestone Conspicuous solution runnels (rinnenkarren) have formed on the top of this sandstone erratic despite its low solubility County Cavan, Ireland Figure Abundant solution pits, typical of lacustrine karst, on limestone surfaces exposed in the zone of seasonal submer gence Lough Mask, Ireland dissolution on a small scale Certain algae and invertebrates adopt an endolithic habit, living within cavities that they excavate in the rock Cyanophytes constitute a significant karstic agent in some climates Where created with acids, these cavities are true biokarst (Figure 4) However, many organisms excavate cavities mechanically, by bioerosion, and these are not strictly biokarst Algal biokarst has been termed phytokarst and is ubiquitous in intertidal marine environments, where endolithic organisms are the dominant karst agent In cave entrances or beneath overhangs, light-orientated phytokarst pinnacles, or photokarren, may develop where biokarstic algae are illuminated from just one direction Medium-Scale Karst (Karst Landforms) Medium-scale features (metres to kilometres) can perhaps be grouped together as karst landforms and develop mainly over time-scales of thousands to hundreds of thousands of years In any karst landscape, dissolution is greatest near the surface where water first comes into contact with the rock, creating a shallow zone of epikarst above less karstified rock Over time, and dependent on climate, the depth and dissection of this epikarst zone increases The most characteristic surface karst landform is the doline (sometimes also known as a sinkhole, especially in North America and by engineers everywhere), an enclosed depression from which drainage is underground; on non-karst lithologies such depressions would form lakes Dolines can form by one or a combination of mechanisms, e.g., by enhanced dissolution through focusing of drainage, by collapse into a cavity beneath, or by downwashing of a weakly consolidated soil or sediment cover into cavities beneath (Figure 5) Collapse dolines may intercept the regional water table to form cenotes; they are particularly characteristic of some lowland karst plains Irregular, flat-floored depressions may develop by the lateral solution and planation of limestone inundated by seasonal rises in the regional water table Known as poljes in their type area of the Dinaric Alps, they are fed by springs and drain via sinks, or may fill and empty via the same conduit, known as an estavelle, depending on flood stage or groundwater level Drainage in karst terrains is not exclusively subterranean and elements of fluvial geomorphology are common Valleys and gorges may develop where rivers cross from adjacent non-karst (allogenic) rocks and travel across the limestone surface before sinking