Grotte Dordogne | Geology | Les Grottes de Maxange
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The cave that has been opened up forms part of an underground system that was formed at the beginning of the Tertiary era, in other words, about 60 million years ago. Between that era and our own, the cavity evolved through a series of changes, which is even now quite difficult to piece together. What one can say is that the flow of water that carved out the galleries and deposited alluvia slowed down with the deepening of the river-valleys. Following climatic changes, the cave was subjected to complex phases of in-fill by sand and clay, washed in from the exterior, alternating with phases of further carving-out. The concretion visible today dates from the latter period, spanning thousands of years, and can only have been formed in those voids that were not affected by sand and clay in-fill.

Profusion of eccentrics for a show of rare quality.

What makes for the importance of this cave is the quality of its concretion. This is the term used for mineral deposits that form on the walls of a cave by crystallisation of calcite (calcium carbonate) carried in solution by infiltrating water. When this water drips from the roof of a cavern, it deposits a small amount of the calcite that was dissolved in it. Over the centuries, this produces a more or less cylindrical cluster of calcite crystals that forms downwards, which is called a stalactite. Similarly, a deposit is formed on the floor of a cavern when a droplet of water falls, forming little by little what is called a stalagmite. One can understand that, following the intensity of the infiltration of water, the shape of the galleries and other conditions, these concretions can take on extremely varied forms, in the shape of columns, draped fabric, etc.

The shapes of concretion formed by calcite which are both the most rare and the most curious are those referred to as eccentric. This type of concretion forms finger-like patterns, twisting in all directions, seeming to defy the law of gravity. It is made possible by a combination of several factors: the particular porosity of the rock, and a constant but very slight flow of water through the roof of the cave. In this way (and to simplify a little) the water, laden with calcium salts, evaporates against the cavern wall without falling to the ground (furthermore, that leads to the formation of very few stalagmites in the cavern). The water weeps slowly, penetrating all the porous sections of the cavern wall and of the concretions that have already formed. The water is subject to the force of capillarity, on this scale greater than the force of gravity, and can climb just as well as it can descend. In this way it deposits particles of carbonate in every possible direction, seeming to defy, as one often says, the law of gravity.

The Maxange Caves are one of the finest examples of what is referred to in geological terms as a “geode”.

This effect is particularly astonishing and can only be observed in a fairly small number of caves where the criteria of the porosity of the limestone, the flow of water filtering onto the walls and the micro-climatic conditions of the cavern all coincide to allow their formation.It is precisely this type of eccentric concretion that one may observe in the Maxange Caves. They form bouquets that decorate the roof and walls over a considerable distance. Their profusion, delicate form and purity, sparkling under rays of light make a display of rare quality, which fully justifies the project of development.