SEDIMENTARY PROCESSES/Karst and Palaeokarst 683 Figure Relict karst towers, accentuated by marine undercut ting, in Halong Bay, Vietnam These testify to active tectonic uplift in a terrestrial environment, prior to drowning by sea level rise or regional subsidence Photo by Sally Hunt perhaps the most spectacular of all karst landscapes, is dominated by often precipitously steep limestone towers, from tens to hundreds of metres high, and is restricted to areas in south-east Asia and the adjacent islands (Figure 7) Cone and tower karsts develop where rates of tectonic uplift exceed, for the former, or are equalled, for the latter, rates of surface lowering at points of focused dissolution, leaving residual hills or towers between In contrast, prolonged denudation with minimal uplift will, ultimately, produce a low-relief corrosion plain, as typified by the Gort lowlands to the south of Galway, Ireland Various classifications describe stages of ‘maturity’ in karst landscapes; the most useful, and comprehensive, scheme (Figure 8) was developed primarily for engineers and encompasses a range of parameters, including karst class (a qualitative measure based on the scale and diversity of karst landforms), mean doline density, rockhead relief, and typical cave size Karst Denudation and Landscape Development In landscapes encompassing both soluble and insoluble lithologies, differences between erosional and dissolutional processes have a considerable effect on landscape development in tropical and temperate regions Karst dissolution is a low-energy process that may occur even in static water and, in the case of limestone, is enhanced by a vegetation cover, which increases soil CO2 levels Analogous weathering of non-karst rocks may alter their composition and mechanical strength without significant volume change or transport, forming saprolites sometimes many metres thick These are removed only by erosion, a higher energy process requiring fast water flow but hindered by vegetation, which binds and protects weathered rock and soil In temperate and Figure Typical landforms encountered within the five cat egories of ground conditions recognized by Waltham and Fookes in their 2003 engineering classification of karst tropical environments, limestone may be removed at a steady rate, proportional to mean annual rainfall, whereas erosion of rock and saprolite is more sporadic and depends on the frequency of intense rainfall events and the extent of vegetation cover In arid regions, limestones commonly prove more resistant to erosion than other, less well-cemented, rocks Conversely, in tectonically stable humid climates, the long-term denudation rate of limestones may be significantly greater than is the overall erosion rate on adjacent rocks, reducing limestone outcrops to lowlands flanked by hills of more slowly denuded lithologies Karst Drainage A significant element of drainage in karst landscapes is underground Drainage into a karst aquifer is either