WORK IN PROGRESS (Part 1) EarthCache
WORK IN PROGRESS (Part 1)
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Dan yr Ogof is a limestone area which demonstrates many features of a KARST landscape. It also has a 17 kilometre long cave system. This earthcache is about KARST landscape and the impact of water on relatively soft limestone rock.
KARST topography is a geological formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering-resistant rocks, such as quartzite, given the right conditions. Due to subterranean drainage, there may be very limited surface water with no rivers or lakes. Many KARST regions display distinctive surface features, with cenotes, sinkholes or dolines being the most common. However, distinctive KARST surface features may be completely absent where the soluble rock is mantled, such as by glacial debris, or confined by one or more superimposed non-soluble rock strata. Some KARST Regions include thousands of caves, although evidence of caves large enough for human exploration is not a required characteristic of KARST.
Often evident in theses landscapes is disrupted drainage networks where streams and rivers often cannot flow above ground continuously. Because the land beneath karst topography is very unstable, it has a tendency to become too fragile to support the surface, and will collapse, creating a sink hole. Sink holes make building or living on karst topography very dangerous. This unstable land can cause massive sinkholes and other geomorphic hazards. Click here to see a recent tragic example in Florida.
Getting here
Travelling from the South (M4)
Leave the motorway at junction 45, and follow the signs for the A4067 North (Pontardawe). You will then see the brown tourist signs for Dan-yr-Ogof Showcaves. Follow these signs all the way up the valley; from the motorway the journey time is approximately 25 minutes.
Travelling from the North
If travelling from the North, head towards Brecon. From Brecon, stay on the A40, travelling in the direction of Llandovery. You should leave the A40 just before Sennybridge (a left turn), looking for the brown tourist sign for Dan-yr-Ogof Showcaves. This takes you on to the A4067; from Sennybridge the journey time is approximately 15 minutes.
Or set your SAT NAV to SA9 1GJ
You don’t need to enter a cave to complete the cache, but you may find it a worthwhile experience. The National Showcaves Centre for Wales is situated here within the Brecon Beacons National Park, and is a fascinating insight into the natural phenomena of cave formation.
About the rocks
The bedrock at Dan yr Ogof is predominently carboniferous limestone, formed some 315 million years ago. Because the rock has cracks and fissures, water has been able to flow through it, dissolving the limestone and carving out the landscape we see today.
The carboniferous limestone was formed some 315 million years ago, when this area that is now the National Showcaves Centre for Wales lay to the south of the equator and was covered by a warm shallow tropical sea. Shellfish corals and numerous small creatures lived in these waters. When they died their shells and skeletons sank to the sea floor. They were the source of calcium carbonate, the raw material for limestone rock. Over millions of years these calcium carbonate muds and sands have been deeply buried and transformed into limestones. The continents have moved around and mountain chains have been built and eroded. The end result is that Dan yr Ogof is now in the temperate belt of the northern hemisphere, thousands of miles from where its limestone originated.
CROSS SECTION OF KARST LANDSCAPE
About 5 million years ago, during and following the development of the Alps, Dan yr Ogof was in a relatively gentle backwater. Features of the present landscape could be recognised. The sea level was about 200 metres higher than at present. The riverbeds were therefore higher than at present. Caves were forming at these levels.
Then about 2 million years ago the Ice Ages started. Polar ice caps and glaciations in the mountainous regions locked up vast quantities of water on land. This reduced the water in the sea and sea levels fell. The rivers responded by cutting into their beds, thus forming gorges and waterfalls. The rivers running underground in the caves cut new passages at lower levels and abandoned the higher levels. Cathedral cave is an abandoned higher level. The show cave of Dan yr Ogof is formed in one of these lower levels and the river is still cutting its way down to form even lower cave passages.
The limestone is attacked along both 'bedding planes' - the layers of rock themselves - and along vertical fractures present in the rock from the times millions of years ago when south Wales was successively stretched and squeezed as continents collided and split apart. Roof collapse also plays a part in the growth of a cave over many thousands of years. Look around the area for other KARST features similar to those illustrated in the diagram above.
Water pours down the southern slopes of the Old Red Sandstone hills to the north and on meeting the limestone, disappears underground. Because most of the National Park's rocks slope gently towards the South Wales Coalfield, many caves follow this southward dip but they also extend east-west across it until they emerge in one of the major valleys carved through the limestone.
If you like messy projects, try this :
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Wrap some sugar cubes inside a blob of plastercene or clay
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Make a hole in the top and a slightly larger hole at the side so that you can just see the sugar cubes
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Gently add a some warm water ~ continue over a a short period of time
Of course the sugar dissolves and leaves a cavity that would become a cave. This is a simulation of water percolating through a sink hole into cracks in the rock and eroding them. This represents a process which would occur in limestone over hundreds of thousands of years.
Some 800,000 years ago the climate became much colder with heavy snowfalls and the development of ice fields on high ground and glaciers in the valleys. The cold spells were interspersed with relatively warmer periods, in one of which we are currently living. The effects on the cave were important as glacial debris filled by floodwaters when the ice melted. The ice came and went and in the melt times stalactites and stalagmites were formed. This build up of cave formations continues today as acidic rain dissolves away the limestone and then precipitates it as stalactites and stalagmites in the caves. Please look out for a partner earthcache "Work in Progress. Part 2" Click here to go the partner earthcache.
Carboniferous limestone produces this distinctive karst scenery. These areas are dry on the surface due to the permeability of the rock, but have mainly been shaped by the action of water.
The water attacks the many joints and bedding planes in the rock, through the chemical weathering processes of carbonation and solution. There are a number of distinctive features seen in karst scenery areas, both on and beneath the surface.
• Limestone pavements are large areas of exposed limestone. When the overlying rock was eroded the pressure release on the limestone below caused it to crack even more. Hence limestone pavements are characterised by large gaps between the rock, called grikes. The remaining blocks of rock are called klints.
• Limestone cliffs or scars are produced at the edge of the area of limestone. Often near vertical and highly jointed.
• Swallow holes and sink holes are where rivers flow down into the rock. Sink holes are relatively small, whilst swallow holes are larger. Both have been formed either by the constant chemical attack of the water on the joints in the limestone, or by the collapse of a cavern below.
• Caverns are underground caves that have been hollowed out by the action of underground streams and by carbonation and solution.
• Underground streams flow down through the limestone carving out caverns, until the y reach the impermeable layer of rock below the limestone layer. Once at this point the stream flows under the limestone until it re-emerges. This is called resurgence.
• Gorges are created where the roof of a large underground cavern falls in, to create a steep sided gorge with a river running in the bottom.
• Dolines are formed when the roof of a small underground cave falls in. The ground above the cave subsides into it causing an indentation on the surface. Click here for a helpful link regarding this area and the KARST features.
THE CACHE IS CALLED "WORK IN PROGRESS" BECAUSE THESE GEOLOGICAL PROCESSES CONTINUE EVEN TODAY AT AN IMPERCEPTIBLE PACE.
Acknowledgements
“Wales Underground” / South Wales Geological Association
Congratulations to the Day Clan on a very swift FTF.
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1. What are the characteristics of karst topography?
2. Describe the landscape around you and any karst featues that you can see in the immediate vicinity?
3. What are the two requirements for karst topography?
4. How do these two factors work together to create karst landforms?
5. What are the risks to people in Karst landscape?
Please do not include any answers in your log.
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