The geological history

The Gouffre de Padirac lies at the heart of the Causses du Quercy (Quercy Limestone Plateaux), an immense limestone plateau in the region and a perfect example of karst* relief.

The formation of the karst landscape of the Gouffre de Padirac can be summed up in four key stages: the formation of limestone, fracturing, hollowing, and formation of speleothems.

Karst, karstic relief or shaped karstic, designates an area generally rich in limestone rocks dug by infiltration water. There are many underground galleries, caves and chasms. The word "karst" comes from the Kras or Karst region in Slovenia. It is there that the first karst phenomena were described scientifically.

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THE FORMATION OF LIMESTONE

The fact that limestone is present everywhere in the region is explained by a generalised marine invasion that occurred during the Jurassic* Period, about 170 million years ago. During that period, the local landscape resembled tropical lagoons that fostered the accumulation of marine sediments, which in turn gave rise to the limestone. The limestone is mostly made up of calcite minerals.

The sediments are mainly chemical* and biological* in origin, as well as being of continental*origin.

The Jurassic is a geological period that lasted from 201 million to 145 million years ago. By way of comparison, our planet is 4.57 billion years old, and it has undergone a series of upheavals that shaped landscapes and the species we know today. The first fossil traces of life date back 3.5 billion years, whereas modern human beings, Homo sapiens, appear to have emerged about 300,000 years ago. The Jurassic is mainly known due to the presence of characteristic fauna: the dinosaurs.

Chemical origin: the minerals present in seawater can be precipitated directly in the form of small calcite particles. Very often, small marbles called ooliths are formed. The Gouffre can be seen to contain several layers of oolithic limestone.

Biological origin: the fragments of shells and of the skeletons of sea animals, mainly made up of calcite, accumulate on the ocean floor. Some fossils can be observed in the Gouffre de Padirac, but they are few in number..

Continental origin: rivers, wind or glaciers transport previously eroded rock fragments from the continent to a deposition area (lake or sea). Certain layers of limestone observable in the Gouffre are rich in clay.

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FRACTURING

Contrary to received wisdom, limestone is an impermeable rock. Without cracks, water cannot seep through the rock. Thus, fracturing is an essential first step in forming an underground network. Fissures (faults and joints)* are the result of significant tectonic movements that constantly affect rocky areas. Many fissures in Padirac are certainly - but not solely - linked to the formation of the Pyrenees (about 40 million years ago). They may go back to the formation of the limestone plateau 170 million years ago.

Fissure: there are two main types of fissure, called joints and faults. Joints are just spaces between two boulders, whereas in faults, the boulders move relative to one another.

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THE HOLLOWING OUT OF GALLERIES

The process of hollowing out galleries is much more recent than the formation of the limestone. It started just 1 or 2 million years ago, during the Quaternary* Period, thanks to water seeping through a network of fissures. The hollowing out of galleries is the result of chemical erosion* (>95%) combined with mechanical erosion* (<5%).

Quaternary: the Quaternary is a geological period that has lasted from 2.6 million years ago until the present day. As a reminder, the Earth was formed 4.57 billion years ago. Condensing the entire history of our planet into a 24-hour period, the galleries of the Gouffre de Padirac were formed at 11:59 p.m, which is very recent on a geological scale.

Chemical erosion: chemical erosion is the dissolution of calcareous rock by acidic water. Water is the "vehicle" that allows erosion to occur; in reality, it is the carbon dioxide dissolved in water that makes the water acidic, which means that carbon dioxide is the real agent in the reaction. Each raindrop that falls on the Causses is enriched with carbon dioxide and is acidified (the formation of carbonic acid). Carbon dioxide comes from biological activity in the soil, more specifically in the humus layer, in which plants decompose. Carbonic acid can then dissolve limestone.

Mechanical erosion: mechanical erosion (related to the strength of the current) sculpts the rock, but it mainly allows debris from upstream to be leached away and channelled.

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Water circulation also meant that circular hollows formed in the roof of the Lac des Gours. Those forms, called potholes, were formed when the river completely filled the hall. They were dug out by swirling movements of water carrying fine particles (mechanical erosion) and by dissolution (chemical erosion).

CAVE FORMATIONS (SPELEOTHEMS)

When seepage water arrives in a gallery, a change in pressure releases carbon dioxide, and the chemical reaction of dissolution is reversed. Calcium carbonate is precipitated, and returns in solid form. Calcium carbonate most often crystallises as limestone, the main constituent of concretions or speleothems: stalactites, stalagmites, cascades, draperies, etc. Each form is the result of a combination of several factors, for example the speed of flow, the shape of the sides, and temperature. Speleothems are incredible climate archives*.

Climate archive: concretions are fantastic scientific tools, and they are regularly used by geologists to trace back our planet's past climate. Before arriving underground, the drop of water forms a record of various soil and atmospheric properties. It then arrives in the underground network and deposits its calcite. All the information recorded on the surface is conserved underground in the concretion. By analysing stalagmites, stalactites, and other draperies, it is possible to trace back temperatures, amounts of precipitation, type of vegetation, etc., over time. Because of that, concretions are some of the history books of our planet.

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