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.
How limestone pavements form
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. Karst scenery
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. Clints and Grikes
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. Runnels, Pits and Pans As well as clints and grikes, limestone pavements also have a number
of characteristic surface formations. These are known as runnels,
pits and pans and take different forms depending on the structure
of the pavement on which they form.
Runnels are gutter-like channels eroded out of the surface of the
limestone which drain into grikes. The formation of runnels takes
place under a shallow layer of soil. Runnels formed on steeply sloping
limestone are usually close together and parallel, whereas runnels
forming on gentle slopes take dendritic or branched forms which
can be looked at as a miniature river system.
Pits and pans are small scale solution features (i.e. formed by
water and acids dissolving the limestone) found on the tops of clints.
Pits are deep and free draining into the subterranean limestone
drainage system. Pans are shallow, have an impervious base and hold
water. Both of these features also form under shallow soil cover.
Some research indicates that the deeper features can be formed by
humic acid from the root balls of trees resting against the limestone
and dissolving it into the characteristic round bottomed features
of pits and pans.
When solution features on a limestone pavement are exposed due to
soil loss, they become more weathered and ‘sharper’
in appearance. It is possible to differentiate newly exposed pavement
from pavement which has been exposed for longer periods of time
by looking at the sharpness
Limestone pavement and
Karren features
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).
References for this section
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
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Smooth clints and grassy grikes

Sandstone erratic bolders

Karst scenery at Great Asby Scar

Rounded, recently exposed pavement

Pavement shattered by long exposure

Water-filled pans after rains

Runnel on a gentle slope
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