680 SEDIMENTARY PROCESSES/Karst and Palaeokarst respiring plant roots and soil organisms With faster flow, generally associated with increased water depth and/or conduit width, laminar flow gives way to turbulent flow and there is a rapid ($fivefold) increase in dissolution rate This laminar–turbulent transition, or turbulent threshold, is very significant for the development of many karst features Time is a major factor to consider in the development of karst, reflecting dissolution rates Features on a millimetre to centimetre scale may form within a few years on limestone, whereas the largest scale landforms may require hundreds of thousands, or even millions, of years to reach their present form Timescales for karst development on gypsum are much faster, whereas on quartz they are very much slower Surface Karst (Exokarst) At the surface of the Earth, karst features are manifest at scales from tens of kilometres down to millimetres Large-scale karst landscapes and landforms invariably support a diversity of smaller scale features The type and scale of karst features that develop at a particular location are determined by several factors that can be grouped broadly: (1) intrinsic rock properties such as composition, texture, porosity, and fracturing; (2) extrinsic factors, particularly climate (especially the amount of rainfall), specific environment (terrestrial, lacustrine, or marine), the type and extent of plant cover, sediment cover (if any), and tectonism (uplift or subsidence); and (3) time In general, karst is best developed when pure limestones with low porosity but good secondary permeability are subject to frequent, rather than heavy, rainfall Hence karst landforms are better developed in humid temperate or tropical climates than in strongly seasonal ones Duration of exposure to dissolution is critical; for particular intrinsic and extrinsic conditions there is an optimum time-scale for specific karst features to develop Beyond this time-scale, dissolution may continue to modify and, ultimately, even destroy the features Small-Scale Karst (Karren) Small-scale solutional sculpting (millimetres to metres), often termed ‘karren’, develops purely as a function of rock solubility and run off, independently of the underground drainage characteristic of karst terrains The development time-scale for most karren ranges from a few years to tens of thousands of years under optimum conditions On exposed limestone solution pans (kamenitzas) form by ponding of water on level surfaces, while solution flutes (rillenkarren; Figure 1), runnels (rinnenkarren), and ripples Figure Solution flutes (rillenkarren) on gypsum, Devon coast, England These may form on gypsum within a few months or years in a humid climate develop by water flow across inclined surfaces Fractures, commonly joints, act as initial conduits for flow into the karst aquifer and are widened by dissolution to form grikes (kluftkarren) The limestone between grikes may, depending on factors such as lithology, soil cover, and climate, form flat-topped blocks (clints or flachkarren), rounded blocks, domes or teeth (rundkarren), or sharp pinnacles (spitzkarren), or may break up into fragments (shillow, felsenmeer, or truă mmerkarren) Glacially stranded boulders may protect limestone directly beneath them from direct dissolution by rainfall, eventually leaving the boulder perched on a pedestal surrounded by a limestone surface lowered by solution The height of such pedestals provides a measure of the surface lowering rate (Figure 2) Clints, grikes, solution runnels, and solution pans are common features beneath mineral or organic soils, though typically they are more rounded than those formed subaerially Freshwater lake shores support their own unique karren assemblages, strongly influenced by lake water chemistry Small hemispherical solution pits (Figure 3) are particularly characteristic on flat limestone surfaces around lakes, and notches (swamp notches) may indicate former water levels Even if the lake water is permanently saturated with carbonate, the limestone can dissolve if seasonal changes in water level trap air pockets in which condensation corrosion occurs; this process forms upward-tapering tubes (roă hrenkarren) Similarly shaped, but larger, features called bell holes are found in the entrance zone of some tropical caves and are considered to form by condensation corrosion associated with daily changes in air temperature Most karren are essentially abiogenic in origin, but living organisms may contribute directly to limestone