Please be aware that limestone pavements are covered in deep cracks, which are just the right size and depth to break legs and ankles if you slip and fall. Please take particular care when walking on the pavement, keep an close eye on mini cachers and dogs and take EXTRA care if the surface is wet.
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.
Smooth
In this location you will notice that the bedding planes are well separated, in fact there are some places where the depth of the pavement is such that there are no bedding planes at all above ‘ground’ level. Get down and look into the cracks and observe the bedding planes, there will be a question about this. (I describe ground level as the place where the pavement meets the grassy surface)
During its creation the thickness of the layers was decided by the flow of water and the rate at which the debris that was creating the rock was deposited. So the more constant the flow of water and the laying down of animal remains, the thicker the layers is. If there were regular breaks in the flow of these materials or if the material composition changes regularly, the layers would have been thinner.
Rough
At this location you will notice that the bedding planes are plentiful and in some places very thin. Looking into the cracks you will see that some of the pavement looks like a stack of plates. Get down and have a good look at the bedding planes at this location, there will be a question about it.
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.
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).
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 the layer of debris left by the retreating glaciers and also by wind deposits and water/ice action, 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 rock 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.
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
Questions for the pavement, please email or message the answers to me, you are invited to log your find at the same time.
- Please describe the pavement at both the location of the cache and the rough reference point, concentrate on the top of the pavement and the depth of the bedding planes.
- Please describe the clints at these locations
- Please explain the differences between the two locations and try to explain how this came about.
- Please tell me roughly how deep the grikes are at the two locations.
If you feel willing and able please include a photo in your log but make sure it doesn’t give away the answer to Q4
Thanks for visiting the location.