Size:  (not chosen)
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This earthcache is placed on the hill in the center of Yangshuo with a beautiful view.
The karst geomorphy around Yangshuo is an area of irregular limestone formations in which erosion has produced fissures, sinkholes, underground streams, and caverns. This is wide-spread in the region, creating numerous natural scenic spots. Around the river Li we can found over 70,000 karst peaks, called also mogotes - isolated, steep-sided, residual hills surrounded by nearly flat plains
Karst Formation History
Two crustal movements of the earth took place about 200 and 180 million years ago thrusting the limestone sediments out of the sea from its bottom to form a large expanse of land. Through many years of erosion by the wind and rain the karst formation became the hills and rocks with unusual shapes.
Main dissolution mechanism
Karst landforms are the result of mildly acidic water acting on weakly soluble bedrock such as limestone or dolostone. The mildly acidic water begins to dissolve the surface along fractures or bedding planes in the limestone bedrock. Over time, these fractures enlarge as the bedrock continues to dissolve. Openings in the rock increase in size, and an underground drainage system begins to develop, allowing more water to pass through the area, and accelerating the formation of underground karst features.
The carbonic acid that causes these features is formed as rain passes through the atmosphere picking up CO2, which dissolves in the water. Once the rain reaches the ground, it may pass through soil that can provide much more CO2 to form a weak carbonic acid solution, which dissolves calcium carbonate. The sequence of reactions involved in the limestone dissolution are the following:
This is the main dissolution mechanism of calcium carbonate in limestone.
Here was the end of my listing, but reviewer asked me to describe what happened here in my words – just for kids, not for scientest. Good idea, I had really problem to write it. So here is my attempt.
In the beginning there was a huge limestone plate. Like a chocolate table on the desk. There are movements of tectonic plares in the earth’s crust and the table was broken to pieces. Imagine the table of chocolate divided to pieces as if you want to eat it.
Next the water from rain and rivers come. If you put water on chocolate it will start to dissolve it and flow parts of it away. The holes and openings become bigger and after some time most chocolate is out. The same as here. The rests of limstone’s plate are karst hills and mogules of different shapes.
You can ask why the water did not make a deep canyon - the bedrock is too hard. The water took the chocolate, but the desk remained the same. The end of the story is sad – after some time there will be no chocolate at all. But here in Yangsho it is a question of many milion years.
To claim the earthcache you have to climb to the top of the hill. There you can see several large stones from the same material as the hills around. Send me via profile answers to this questions:
1) What is the name of the rock (material, not hill)?
2) Do you see any holes in the stones? (none/few/many)
3) How many separate rocks you can see?
4) Can you estimate how thick was the "chocolate table"?
(No hints available.)