Limestone Features

I

Introduction

Limestone Features, landforms created by the decomposition of limestone through the solvent action of water in a chemical weathering process called solution. Such landforms are most commonly associated with karst landscapes—limestone areas characterized by the dominance of solution processes, by a lack of surface water, and by often spectacular scenery.

Limestone is composed principally of calcite, or calcium carbonate. Calcite is virtually insoluble in pure water, but is highly vulnerable to the carbonic acid formed when carbon dioxide dissolves in water. The carbon dioxide can derive from the atmosphere, through rainwater. In areas where the limestone is covered by soil it can also derive biologically, as a result of plant respiration and bacterial activity; in these situations, the carbon dioxide in the soil reaches high concentrations. In a process known as carbonation, the resulting carbonic acid breaks down the calcite to produce calcium bicarbonate which is soluble in water and readily removed from the rock. The chemical weathering thus produced tends to be enhanced in soil-covered limestone areas.

In common with most chemical weathering processes the rate of carbonation increases with temperature. However, the amount of carbon dioxide that can be held in solution increases as the temperature falls, so that high concentrations of carbonic acid can be expected in the soils of cold regions. Because of these factors, and because limestones are the most commonly occurring soluble rocks on Earth, carbonation is an active weathering process in almost all climatic zones.

Karst landscapes develop best on areas of very thick, well-jointed, and relatively hard limestone. The jointing of the rock is necessary to provide routes along which water can enter and leave the rock to achieve, and remove, the products of carbonation. Another prerequisite for extensive karst development is considerable local relief, to allow the water to flow vertically down through the rocks before reaching the water table deep under the surface. Other calcareous rocks, such as the Cretaceous chalk which underlies much of southern England and parts of northern France, fit some of the prerequisites but are too soft to support well-developed karst terrains. Eastern Slovenia, characterized by typical limestone topography, is one of the world’s best-known areas of karst, and is where the term originated; “Karst” is the German form of the Slovene word “Kras”, meaning barren, stony ground. Other areas where karst scenery is particularly well developed include the Tertiary limestone areas of Croatia, including Dalmatia; the Carboniferous limestone areas of the English Pennines; the Mesozoic limestones of the Causses area of central France; the Burren in western Ireland; Jamaica; New Guinea; and parts of southern China. The karst area of southern China is the largest in the world, and its spectacular scenery (notably the mogotes—large, steep-sided, residual limestone hills) has inspired many Chinese artists.

II

Karst Surface Features

A

Small Landforms

Small-scale solution features, ranging in size from a few millimetres up to about 10 m (33 ft), are known collectively as karren or lapiés. The terms, respectively German and French in origin, originated in the Alps, where these features were first described. Karren are best developed under a soil cover, when they are generally deeper and more rounded than those formed on exposed rock, which tend to be quite sharp edged and relatively shallow.

Limestone pavements are common in many areas, including the Pennine Hills in North Yorkshire and the Burren in Ireland. They are composed of tabular slabs of limestone, known as clints or flackkarren, which are separated by variable-width vertical cracks, called grikes or kluftkarren. Grikes are developed by enhanced solution along joints, and when they have grown to a point where the clint has almost disappeared, the result is a solution spike or spitzkarren. The outer parts of the pavement, which have been exposed longest to the atmosphere, often have quite sharp weathering features. At the back of the pavements, where they emerge from the soil, the clints are generally smaller and have a much more rounded appearance. The exposure of very extensive pavements, such as in the Pennines, may be partly a function of the stripping away of the soil by moving ice during the last glaciation.

Where the surface of exposed limestone, or a clint, is relatively flat it often displays small solution features related to either flowing or standing water. Those created by flowing water are analogous to river channels in that they often meander and usually have increased cross-sectional areas in a down-channel direction. These features are known as solution runnels (or rinnenkarren) and can be up to 50 cm (20 in) wide and deep, and in excess of 10 m (33 ft) long. Where solution runnels develop under soil they are of similar dimensions but much more rounded, and are known as rundkarren. Standing water on flat limestone surfaces can produce small, almost circular, dish-shaped depressions known as solution pans. Once these features have begun to form, soil and other organic material accumulates in the developing hollow, providing additional carbon dioxide from biological sources and thus enhancing the chemical weathering processes.

Where the surface of the limestone is vertical or sloping steeply, for example along the sides of clints or solution runnels, solution flutes or rillenkarren develop. Although associated with flowing water, these features tend to be much smaller than those developed on flatter surfaces, and they do not exhibit increased cross-sectional area down channel.

B

Large Landforms

A characteristic feature of virtually all limestone landscapes is the closed depression, or hollow, which can vary in size up to 100 m (328 ft) in depth and 1,000 m (3,280 ft) in diameter, and is known as a doline or solution hollow. Most dolines develop from enhanced solution of limestone where there is a higher density of faults or joints in the rock, often at locations where two joint patterns intersect. The drainage from the base of the doline may be via pipes or shafts (also called avens and gouffres) in the rock into underlying cave systems. In some cases, however, the drainage may be more diffuse, moving along narrow fissures developed along the joints in the rock. As a doline develops it can capture surface drainage, particularly where it flows off overlying impermeable rocks. The captured stream may then disappear in the base of the doline, in which case it is often called a sinkhole, swallow hole, or swallet. Streams may only reach a sinkhole after heavy precipitation, or in the winter months when discharge levels are high. As the sinkhole and river system deepen through erosion and solution, a blind valley may develop, ending abruptly at the sinkhole.

Other dolines, known as collapse dolines, are formed by the collapse of the roof of shallow cave systems. If the water table is close to the surface, the collapse doline can be occupied by a lake, examples being the cenotes of southern Mexico. Other dolines develop in limestone under superficial soil cover; in the case of northern England this is often glacial till. When this happens they are known as subsidence dolines.

In areas where doline development is extensive, notably in the humid tropics where solution rates are very high, individual dolines can coalesce to form much larger depressions known as uvalas. The process of merging can produce a network of polygon-shaped hollows with surrounding low ridges, known as cockpits, creating a hummocky terrain. Hills on the ridges, representing residuals of the original surface, are known as kegel or cones; where the hills are very steep and sharp edged they are known as pinnacles. This assemblage of landforms is known as cockpit karst or kegel karst, and is very characteristic of Jamaican limestone scenery. Cockpit karst also occurs in New Guinea. Some limestone areas in humid tropical and subtropical areas, for example Guizhou province in south China and parts of Vietnam, are dominated by the very steep-sided, pointed hills known as mogotes. Separated by wide, flat alluvial plains, they produce a very spectacular landscape known as tower karst or turm karst.

In Croatia there are examples of very large hollows in the limestone, often tens of kilometres long and possibly a similar distance wide. These features, which generally have a very even floor and a clear angle between the floor and sides, are known as poljes. They are thought to be tectonic depressions bounded by fault lines. Another large-scale feature commonly developed on calcareous rock surfaces is known as a dry valley. These generally have all the normal characteristics of drainage systems but do not presently contain rivers. Dry valleys represent a major environmental change, since surface drainage must have existed in the past to erode the valley. Such surface drainage could have been supported by higher water tables, by increased discharge allowing the river to maintain its flow across a limestone area, or by permafrost making the ground impermeable. Some rivers have sufficient discharge today to flow across outcrops of limestone, such as the Dordogne in France. They commonly occupy deep, steep-sided gorges. The steepness of the sides is because the permeable nature of the rock leads to relatively slow rates of weathering. An example is the Tarn Gorge in southern France. Some gorges, usually of relatively small size, may be the result of the collapse of the roofs of cave systems. Examples in England include Cheddar Gorge in Somerset and Gordale Scar in North Yorkshire.

III

Underground Karst Features

Caves in limestone originate in the same way as many surface features, by solution along preferred weaknesses in the rock, for example, faults, joints, and bedding planes. Once solution has opened a passage in the rocks, further enlargement can be assisted by erosion, including abrasion by the sediment particles carried by the flowing water. Cave systems that are full of water are said to be in a phreatic condition; those containing water and air are in a vadose condition. Generally, those caves which are at a level below the local water table will be phreatic; those above will be vadose. However, in many limestone areas this distinction is not always clear; phreatic caves often exist well above local water table levels.

The initial development of most cave systems is probably under phreatic conditions, requiring a hydraulic gradient so that there is a head of water to generate flow within the limestone. This head is usually the result of considerable local relief, which gives a height difference between the two ends of the system: the sinkhole at the surface of the limestone, and the point at which water re-emerges from the rock, which is called the resurgence (for example, Malham Cove in North Yorkshire).

Phreatic caves almost always develop along joints or bedding planes in the limestone. They often have a circular or elliptical cross-section, and the surfaces of the cave walls are frequently quite smooth and rounded. The continued development of such systems depends in part on the hydraulic gradient being maintained, or increased, by the steady lowering of the point of resurgence. This usually involves erosion of the valley in which the resurgence exists. The process of maintaining the hydraulic gradient also encourages the drainage of the upper parts of a phreatic system, so that air enters the caves and they become vadose. When this happens, a channel is often cut into the floor of the rounded phreatic cave to give a keyhole cross-section.

Once the cave has become vadose, stalactites (growing down from the roof) and stalagmites (growing up from the floor) can develop. These result from the evaporation of drops of calcium-rich water, leaving columns of almost pure calcite. Stalactites and stalagmites often develop spectacular forms and can become coloured by impurities, commonly iron, carried in the water. Suitably illuminated, they become the main attractions of show caves.