Geology and geomorphology

Limestone pavement is a habitat with a high geological interest. Forms of limestone pavement can be found in many places in the world, especially in Alpine and Mediterranean areas, but these lack the distinctive surface patterning seen on British pavements.

Limestone is a hard sedimentary rock consisting of calcium carbonate, formed by the deposition of plant and animal remains on the sea floor and is thus known as a calcareous rock. Limestones often contain the visible remains (fossils) of shells and corals. As limestone is a sedimentary rock, it is laid down in layers or ‘beds’ separated by ‘bedding planes’ which are caused by changes in deposition rates or content of material deposited. Limestone pavements in England, Wales and Ireland are mainly formed on deep beds of Carboniferous limestones which were deposited about 350 million years ago. Those in Scotland can be found on more ancient Durness and Dalradian limestones.

The formation of limestone pavements in the UK and Ireland began with the scouring of the limestone by kilometre thick glaciers during the last ice age. The weight of the ice removed the soil that lay over the limestone, and also fractured the limestone along existing horizontal surfaces of weakness known as bedding planes. Fractured rocks were stripped away leaving level platforms of limestone on which a thick layer of boulder clay (glacial till) was deposited as the glaciers retreated. Wind blown material was then deposited on top of the boulder clay. This external material is particularly important for soil formation, as limestone does not weather down into soil which would mean if there had been no glacial deposits, there would have been no soil development. From the flat limestone surfaces, the characteristic features of limestone pavement have been formed by water in the glacially deposited soil exploiting cracks and fissures in the rock such as bedding planes and joints (lines of weakness in the rock generally running at 90o to bedding planes).

Also deposited on the limestone platforms were erratic rocks, ranging in size from pebbles to huge boulders. These erratics, often rocks of a different type to the limestone, are one of the most visible indicators of glaciation on limestone pavements.

Limestone pavement is a type of karst landform. Karst is the word for an area of soluble rock in which the landforms are of a solutional nature (i.e. they are caused by water dissolving the rock ) where drainage is usually underground through rock fissures rather than in surface streams. Karst, a term originating from a limestone region in the former Yugoslavia, is derived from the Slovenian word kras, meaning a bleak, waterless place. It is used to describe a landscape containing erosional features such as bare pavements, subterranean water systems, dry valleys, dolines which are small to medium sized closed depressions with no surface outlet for water (hundreds of metres in diameter and tens of metres deep); and poljes which are large depressions with diameters of 1km upwards and depths of over 50m or so.

It is the layer of debris left by the retreating glaciers and also by wind deposits that hold the key to the development of the characteristic limestone pavement landforms over the last 10,000 years. The karst landforms were formed by the gradual dissolving of the limestone by mildly acidic water seeping through the forest cover which had become established on the soil above the limestone platforms. If this glacial debris had not been in position, then the weathering processes that form limestone pavements could not have taken place.

Due to the solubility of limestone, limestone pavements are associated with some very curious and unusual landforms. The most characteristic surface feature of limestone pavements is their division into blocks, called clints, bounded by deep vertical fissures known as grikes. Clints and grikes form under relatively deep cover of soil where water, carrying carbonic acid which is formed from dissolved carbon dioxide as well as organic acids from decaying vegetation, picks out vertical lines of weakness (joints) in the rock. These fissures widen over the years as the acidic water preferentially attacks the lines of weakness. Grikes take many thousands of years to form under the soil as the rate of solution is slow.

Over time, the soil on the top of the limestone platform began to disappear down the newly eroded grikes, and was gradually eroded from the tops of the platforms. Some of the material lost into the grikes was washed deep into the drainage systems of the limestone pavements through connecting fissures, leaving open grikes of a metre or more in depth. These erosional processes were increased when forest clearance and grazing was introduced by humans onto the buried pavements leading to a more rapid exposure of pavements.

The limestone features described above have specific scientific names and are known as ‘Karren’. Karren is a general term used to describe the total complex of superficial micro-solutional features of soluble rocks such as limestone and gypsum. Karren are particularly common on limestone pavements. Karren forms include sharp-ridged grooves (rillenkarren) and their larger, elongated cousins (rinnenkarren), as well as rounded runnels formed beneath a soil cover (rundkarren). Other forms include the ubiquitous solutional hollows (kamenitzas), grikes (kluftkarren), clints (flachkarren), and horseshoe shaped stepped structures (trittkarren).

Burrenbeo website (www.burrenbeo.com)

Dunford, B (2002) Farming and the Burren

Waltham A.C., Simms M.J., Farrant A.R and Goldie H.S. (1997), Karst and caves of Great Britain, Geological Conservation Review no 12, JNCC, Chapman and Hall

Webb, S and Glading, P (1998) The ecology and conservation of limestone pavement