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The formation of limestone
The deep valleys, the great cliffs forming natural shelters and used by men since prehistory, or the numerous caves, all result from a general evolution - the erosion of limestone which has been given the name of Karst.
To fully understand the various steps leading to the formation of limestone means going back millions of years.
The most ancient rocks appeared 3,7 billion years ago.
The Coniacian limestone that composes the cliffs of the Vézère valley formed 88 million years ago during the secondary era or " Superior Cretaceous " when the region was covered in a temperate and shallow sea. Sediments accumulated on the ground to form limestone mainly composed of sand, marine and clay shells and possibly up to 60 metres thick.
At the end of the secondary era, 65 million years ago, the sea receded further west leaving Périgord high and dry.
During the Tertiary Era, all elements necessary for the formation of caves were united: limestone cracked under both the action of the tectonics and under the weight of the rock, sand and gravel coming all the way from the Massif Central covered it. Erosion and infiltration could begin their work.
drawing of water flowing within the limestone cliff.
When flowing through the plateau, rain waters become acid. They finally erode the limestone cliff, and carry the dissolved limestone. When reaching the cave, they deposit carbons in the form of calcite.
Chemical erosion: the limestone cliff acts as a sponge: as infiltration waters flow through vertical cracks, they collect carbon dioxide and small particles of clay, and the acid erodes limestone.
Mechanical action: cracks become wider and meet, thus forming horizontal galleries. Important amounts of water can now flow into the cave and start eroding the rock.
Each drop of water going through the rock contains carbon dioxide, the acidity of which dissolves limestone.
When the drop enters the cave, the mechanism is reversed: due to the changing pressure and mainly to the important drop in temperature, carbon dioxide escapes and part of the water evaporates.
Carbonates then precipitate to form calcite crystals.
This principle is simple but quite variable, which explains why crystallizations can differ so greatly.
The cave of the Grand Roc contains all existing forms of crystallizations, from the simplest to the most surprising.
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Stalactites
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They grow as a function of gravity, from top to bottom. Water generally comes through a central canal but when this canal is blocked or when the flow is very important, water then flows along the stalactite, and the stalagtite thus enlarges. Only the calcite deposit formed by falling drops makes it become longer. |
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Stalagmites
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Water falls down from the ceiling or from stalactites; when it still contains enough carbonate, it forms a calcite deposit on the ground. The thin calcite layers thus piling up form a small bump, then a concretion which grows upwards little by little. |
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Stalagmitic columns
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When both a stalagmite and a stalactite develop at the same time and place, they meet to form a stalagmitic column, which then stops lengthening, but keeps enlarging. |
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Fistulous concretions
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Thin vertical concretions with a central canal. They are made of extremely pure calcite. Their development requires three main elements: a veil on the cave wall, a central supply canal in the rock, and a slow and regular flow of water. |
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Eccentrics
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Concretions growing in all directions, and not influenced by gravity. The cave of the Grand Roc contains many examples. They probably belong to the same group as fistulous concretions, but the way they develop remains unclear. However, they might result from micro-draughts or from the presence of impurities in water. |
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Rock Pools
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Small natural dams forming onto a stalagmitic ground due to the accumulation of calcite around small impurities or around flaws in the ground. With time, they can even look like small stairs. |
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Triangles
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They develop on stable submerged calcite ground and owe their form to the geometry of calcite crystals that compose them. |
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