Secrets of Pech Merle
Once upon a time… Pech Merle
Geological history of the cave
The authors :
Thierry Pélissié, docteur en géologie, professeur de Sciences de
la Vie et de la Terre, responsable du site des
phosphatières du
cloup d'Aural, Bach, Lot, France.
Serge Roussel, chargé de mission au musée de Préhistoire de
Cabrerets (Pech Merle), Lot, France.
Acknowledgements for the translation in English :
- Kristina Jacobson
- Ann Lewis LLoyd
- Ruth G. Lyons (Department of Physics, The University of
Auckland, New Zeland)
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.
The hot climate and the clear waters bring to mind the present-day bahamas.
The sedimentation areas during the early and later Jurassic periods
(illustration : extract from SPELUNCA n°73)
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.
The limestone was formed on (1) the impermeable marls of the early
Jurassic period.
How was the limestone formed ? By a chemical reaction
(2).
The rivers running down from the current "Massif Central" transport dissolved calcium carbonate (3).
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 grow (4).
These plants absorb the carbonic gas, which causes the precipitation of
calcium carbonate (5). 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.
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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.
180 million years ago
Cross-section
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. .
150 million years ago
The limestone level in which the Pech Merle cave is situated was formed
135 million years ago
135 million years ago
the sea withdraws from the area which is to become Quercy. It is not really that the sea becomes less deep; it is the relationship between the level of the sea and the level of land that changes.
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 million years ago
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.
65 million jears ago
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.
50 million years ago
An example of how water could flow through a limestone massif with
a
canyon-type relief :
1. absorption zone
2. water flow zone
3. water flow zone, submerged at times
4. permanently submerged zone
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).
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.
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.
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.
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.
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.
Today, as result of successive surface erosion, the thickness of the limestone above the cave varies between 10-40 metres.
