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.
The River Wharfe begins high on the Pennines in a peaty bog known as Cam Fell.
From here many springs arise, forming streams.
The faint lines show where streams are flowing down the slope. These form Oughtershaw Beck.
This flows on, through a deepening valley, to Beckermonds, an isolated hamlet (that’s a place that is too small to be considered a village). Here Oughtershaw Beck joins with Green Field Beck.
The combined river becomes the River Wharfe.
And then it disappears!
Well ok it depends on how much water there is to start with; how much rain there has been. It may not all disappear but some of it certainly does as there is less water flowing along the next stretch. It had rained a lot when I was there but this is what can happen.
At Beckermonds, the River Wharfe flows over limestone rock. Limestone has lots of cracks in it and some of the water disappears down these. If the weather has been dry, and the river flow is already low, then the whole River Wharfe can disappear underground. It usually re-emerges about 3km downstream near Yockenthwaite.
‘Wharfe’ is a Celtic word meaning twisting or winding and the River Wharfe certainly does plenty of that. Next week we will take a look at some of the Wharfe’s twisting meanders and discover features that you can look out for on any river.
Meanwhile sign up for the Blog About Britain newsletter and choose ‘Geography worksheets and ideas for further study’ to get a map of the places mentioned in today’s blog.
With dry valleys on the surface and swallow holes swallowing rivers, it makes sense that there are rivers flowing somewhere underground.
Underground stream passages tend to be narrow and winding and can fill completely with water which makes exploration difficult.
In order to find out which swallow hole links to which spring, dye testing can be used. Any colouration is rapidly diluted away by the river but the chemicals can still be detected at likely exit points.
The results are not always what you would expect. Downstream from Malham Tarn the water disappears at the appropriately named Water Sinks. If you continue down the dry valley you come to Malham Cove, at the base of which is a spring. This was thought to be the re-emerging river but dye testing has shown that most of the water from Water Sinks re-emerges further down the valley at Aire Head Springs. Most of the water emerging at Malham Cove is from the Smelt Mill Sink further to the west.
Rivers flowing through the ground can cross over each other on different levels. You need to think in 3 dimensions in order to understand where they flow and how they relate to each other. If you’ve signed up to receive worksheets from me then I’ll be sending you a map to help you make sense of the situation at Malham. If you want to sign up the form in the sidebar.
You usually find a river flowing along the bottom of a valley.
Water heads down the side slopes and then flows along the valley in the downhill direction. As it does so, it removes soil and other loose material so that, as time passes, the valley gets wider and deeper.
But in areas of limestone rock you are likely to come across valley shapes without the river.
If you’ve been following this series, you will already know that limestone is full of cracks, which get enlarged as the limestone dissolves and the water disappears into the ground.
But if the rock is limestone, how did the valley form in the first place?
Sometimes a dry valley is formed because an underground passageway caves in. The resulting valley is likely to slope rather randomly and may not have a clear downhill direction.
But many dry valleys look as though they should have a river in the bottom because they were shaped by a river in the past.
The holes in limestone gradually enlarge as the water flows through and more rock is dissolved. So turn the clock back a bit and the holes would have been smaller: maybe not big enough for the whole river to flow into the ground. So some water could have remained on the surface, cutting the valley shape as it flowed.
This is not the only possible explanation. What if the limestone had plenty of holes but they were blocked for a time. Can you think how this might happen?
If you are in an area of limestone rock, then it is quite common to encounter a rather random depression in the land surface. This might be like a shallow bowl or dish or it might be more like a funnel disappearing into the ground.
These features have a variety of names depending on their size, shape and location, but are pretty much formed in the same ways.
Large bowl-shaped depressions, ranging from several metres to several hundred metres across, are often called dolines.
Water moving through the ground and dissolving limestone creates holes under the surface. These can either collapse catastrophically or, more often, the loose material lying on top of the limestone is gradually washed down into the holes and carried away by the water. (Rather like sand moving through a sand timer. Set one up and watch). The surface gradually collapses forming a doline and, once the bowl shape is formed, both water and soil will tend to move towards the bottom, maintaining the flow into the ground.
A funnel shaped hole is more likely to be called a sinkhole or a swallow hole (especially if a river is disappearing down it).
When a river flows onto an area of limestone, it will start to seep into the river bed, enlarging the passageways into the ground as it goes. Eventually the whole river could disappear into the ground. If you look at a map of a limestone area you can often see streams that just seem to end. They end on the map because the water has dropped below the surface, down a swallow hole.
On limestone these features form naturally but they are features of the man-made environment too. We encountered doline-like depressions in the flashes of Cheshire where the rock had been deliberately dissolved to extract salt.
Occasionally sink holes in towns hit the news. They are often the result of water pipes leaking into the ground and washing away the material around them. Eventually the hole gets too big and the surface collapses.
Last week, in the Burren, we met Clint and Grike. Today I want to introduce you to Kamenitza and Karren.
You will have noticed that although the clints have a relatively level surface overall, if you look in detail they have many little pits and small channels on them.
As you can see here, the depressions collect water. Rainwater is naturally slightly acidic so is able to dissolve the limestone, causing the depressions to gradually get bigger, meaning they collect even more water and so on. The depressions are known as kamenitza.
When rain falls on limestone pavement, some collects in kamenitza, while the rest of the water flows to the edge of the clint and disappears down the grike.
The flowing water will also dissolve the limestone beneath it, so where the water flows most, will be dissolved most, forming a channel. These channels are called karren. They are easiest to spot where they go over the edge of the clint.
Join me next week to look at some of the larger limestone landforms.
Meanwhile, on Saturday, on the Blog About Britain Facebook page, I’ll be starting a new competition based on today’s post!