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Once upon a time… Pech Merle
Geological history of the cave
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The authors
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Acknowledgements for the translation in English
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| 180 to 135 million years ago,
the limestone plateaus of Quercy are formed with thickness of 750 metres. In those days, the continents are very different from today. During the Jurassic period, when the central Atlantic area and the Thetys sea open up, a shallow sea covers the area which is to become the Quercy region. |
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The hot climate and the clear waters bring to mind the present-day bahamas.
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Infra-tidal zone : under the low-tide level (always submerged). Supra-tidal zone : above the high-tide level (always exposed). Inter-tidal zone : below the high tide, above the low tide. |
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The sedimentation areas during the early and later
Jurassic periods |
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| The limestone was
formed on1 the impermeable marls of the early Jurassic period. How was the limestone formed ? By a chemical reaction2. The rivers running down from the current "Massif Central" transport dissolved calcium carbonate3. The zone closest to the coast is a warm and shallow sea (max 30 metres deep, zone b in the illustration). In this zone seaweed-like plants containing chlorophyll and using photosynthesis grow4. These plants absorb the carbonic gas, which causes the precipitation of calcium carbonate5. The limestone is deposited on the sea bed in several layers, in the form of carbonated mud. Finally, the pressure causes cementation and at the same time compress the sea floor. The deepth of the sea remains around the same level. _______________________________________ 1. There is a continuity between the formation of the marls and the limestone. The marls are formed in the deepers areas, such as zone a, and as the sea bed rises, the limestone is deposited on it. 2. Limestone formed by means of the accumulation of the remains of micro-organisms and shellfish is rare in Quercy (fossils are rare). 3. Rocks such as granite contain minerals (mica and feldspar) and soluble calcium ions. 4. The synthesis of organic compounds from carbon dioxide and energy provided by sun light. 5. Biochemical precipitation.
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180 million years ago
Cross-
Most often, in the Quercy region, during the formation of the limestone, one can see a variation between shorter periods of deposition, and longer periods with no deposition. The limestone layers (strata) correspond to the "shorter" deposition periods, whereas the layers in between correspond to the longer periods.
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150 million years ago
The limestone level |
limestone
- 750 m ------ |
135 million years ago
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135 million years ago, the sea withdraws6
from the area which is to become Quercy. 135 to 95 million years ago, the
limestone is modified by the tectonic movement of the Earth's crust and by
erosion. The rock is fractured and the limestone becomes thinner. |
95 to 65 million years ago, a shallow sea once again
covers the area. Gritty (consisting to a large extent of sandstone) and/or
chalky limestone is deposited to a thickness of some 180 metres. |
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95 million years ago
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---- ground level
limestone
impermeable marls |
- 180 m |
65 million years ago
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---- sea bed
limestone
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65 million years ago, a general lowering of the sea level occurs. The sea
withdraws permanently. At the same time, the region corresponding to
present-day Quercy rises. Over a "short" period of time (around 500,000 years), surface erosion and the hollowing-out of a number of caves (among other Pech Merle and the "phosphorites caves") occur. The region has tropical climate and vegetation. 400-500 metres under the surface, the limestone is permanently submerged under water. |
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more than 50 million years ago
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An example of how water could
flow through a limestone massif with a
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| The hollowing-out is achieved by means of chemical action caused by rain. Water rich in carbon dioxide after coming into contact with the soil (microorganisms such as bacteria) dissolves the limestone. This occurs primarily in places where the water circulates easily, i.e. where the limestone layer is broken off (planes between two layers, breaks). | |||
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Cavities hollowed out in permanently submerged limestone look like rounded
pipes or tunnels (called pressure-flow tubes) with dome-like formations. The hollowing-out, the water receding and a first phase of limestone formation, all this is achieved during about 500,000 years. |
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| The
cave is then entirely filled with sand, which comes from the dissolved
gritty surface limestone. The cave remains filled during the period 60 to 1.5 million years ago. During this time, surface erosion continues, if somewhat irregularly. |
The layers of limestone are permanently waterlogged and the sand-wrapped formations are heavily corroded. | ||
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| Exterior erosion creases, especially during the period between 20 and 2 million years ago. The limestone is again covered by formations of clay-type limestone, marls, deposited in lakes and swamps (present day areas of Quercy Blanc). | 2 million years ago, the region starts to rise. Softer surface layers are carried away by erosion. The valleys are formed, e.g. Lot, Célé, Sagne. | ||
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about 20 million
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Water begins to flow again through the cave,
more or less regularly, more or less energetically. Beginning 500,000 years ago, in several spells, there is a partial evacuation of the sand. The dripstone floors are formed, some vaults, rock faces, floors and columns crumble. The "newer" formations start to take shape : stalagmites, stalactites, columns, hanging discs, pearls, etc. |
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With the lowering of the valleys, the natural (prehistoric) entrance to the
cave's lower level is formed. Animals penetrate the cave, which becomes their seasonal habitation : hyenas, "lions" and, above all, bears. Between 30,000 and 10,000 years ago, paleolithic hunters enter the cave at various periods. |
10,000 years ago, the latest Ice Age ends and
the prehistoric entrance is blocked by an earth scree.
Today, as result of successive surface erosion, the thickness of the limestone above the cave varies between 10-40 metres. |
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