Last week we looked at some of the issues involved with extracting limestone from the landscape. But why are we so keen to get hold of this stuff anyway?
Limestone has been quarried in Britain since at least Roman times. Stone walls make use of odd shaped pieces, while more carefully shaped blocks are used for buildings.
Portland limestone, from Dorset, has been used for many notable buildings, including St Paul’s Cathedral.
In limestone areas you will often find old lime kilns, because it was discovered that burning limestone produced another useful substance. Lime could be used as a mortar to fix blocks together, while sprinkling it on farmland results in better crop growth.
In industry, limestone is used in the production of cement and concrete and to help remove impurities from iron, in the making of steel.
It is often under your feet, in road construction, paving slabs and carpet backing. It is used as a filler in paint, sealant and glue. It is consumed in animal feed and also by us, in food such as bread and pills.
Use this link to find lots more information.
Limestone then has a multitude of uses, so it seems that quarries are necessary scars on our landscape.
Limestone has a multitude of uses (as we will discover next week) but in order to make use of it, the limestone needs to be removed from the ground. This is usually done by quarrying.
The rock is dug out from the surface gradually creating a bigger and bigger hole, often surrounded by cliffs.
A quarry is a controversial addition to a scenic limestone landscape. It is likely to result in more traffic, as well as creating dust and noise. But on the plus side, roads may be improved and jobs become available. The workers are likely to spend some of their wages in other local businesses too.
The visual impact can be reduced by screening with trees while the quarry is worked, and then replanting and landscaping after the stone has been removed.
The land may then be left for nature to reinvade or it might be put to use in another way.
My picture was taken in winter, but the base of this old quarry is a busy caravan site in summer. A nicely sheltered spot, on what would otherwise be an exposed hill top.
What does limestone have in common with these?
Both of them are mainly made of the mineral calcium carbonate.
Limestone is formed most readily in warm shallow waters, which is the environment that shell-dwelling sea creatures love. When these animals die, their shells get tossed and broken by the water, but the bits gradually pile up on the sea bed.
As the amount builds up, pressure compacts it, fluids are squeezed out and crystals reform until everything is cemented together, forming limestone.
So limestone is formed from sediment – calcium-rich sediment. This makes it a sedimentary rock.
Limestone is usually a greyish rock but the colour varies depending on what else is present when it is being formed. The sea environment will vary from place to place. There may be sand washing around or a river might flow out nearby and bring mud. Also changes in the environment cause different layers in the limestone. You can usually see these, though they may no longer be flat.
So next time you are in a limestone area, see if you can see the layers. And also have a look for fossils. You can often spot bits of shell.
A turlough or turlach is a limestone feature that is almost unique to Ireland.
The name comes from the Irish word tuar, meaning dry, and the feature is a disappearing lake. The lake fills from water rising up through the holes in the limestone ground and it later disappears in the same way, back into the ground.
Some turloughs result from a period of heavy rain. The underground passageways become full of water and some of it comes out above ground and sits on the surface for a while, until water levels drop again.
Other turloughs are seasonal, filling up in the autumn and draining away again in spring or early summer.
Where the cave system ends under the sea, turloughs can be controlled by the tide, with the water level in the lake rising and falling as the tide comes in and out.
Most touloughs flood to just a few metres in depth and Ireland’s largest covers about 1 square mile.
Turloughs are common in Ireland but rare elsewhere. This is because in Ireland the limestone forms low land close to sea level, whereas usually limestone tends to form higher land, leaving plenty of space for water to drain down through the rock, without all the passageways becoming flooded.
There is a turlough in the UK, at Pant-y-Llyn, near Llandeilo, in South Wales. It is seasonal in nature, filling in late autumn and emptying again by June.
Do you know your stalactites from your stalagmites?
They are both formed as water drips from the ceiling of a cave, with stalactites hanging from the ceiling and stalagmites growing up from the ground.
There are various different ways of remembering which is which, such as stala ctites spelt with c for ceiling and stalagmites having g for ground, but if I put a photograph in sideways, could you tell which you were looking at?
What about this one?
Do you see the difference?
Water flowing through limestone carries in it the dissolved mineral calcium carbonate. If the water trickling down through the cracks in the rock comes to the roof of a cave, it drips down and, as it does so, some of the calcium carbonate is left behind. This repeats over and over. The water drips from the point each time, until a finger of calcium carbonate hangs like an icicle from the roof.
Where the drip lands, more calcium carbonate may be deposited, only this time the water is landing in a splat and the deposit can be more spread out. The formation is thicker forming a chunky pillar.
My photos are from Pooles Cavern in the Peak District, where the stalagmites look rather like poached eggs due to an unusual colouration which has not been identified.
You can take a virtual tour via their website. Take yourself right to the far end of the cavern, where there is a boulder-choke blocking the route through to further caves. The boulders right next to the walkway have been cemented together by deposited calcium carbonate making a very peculiar feature. Don’t forget to look up and down as you make your way through. Use the compass to help, if you get disorientated.