Limestone Pavement

An 'Exmoor Ponies in Conservation' project

There is about 1 hectare of limestone pavement on Myers Allotment

Limestone Features

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.

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.