> CAVE-FILL
Nature abhors a vacuum. One might also say that Nature abhors the caves hollowed in limestone formations. A study of karsts and their evolution shows that the empty spaces created in the rocks are inexorably fated to be filled in. Under ground as above, the circulation of water brings in elements and minerals, dissolved, in suspension or carried along by the flow, and under certain conditions releases them. The accumulation of these deposits fills up the hollows and is known as "cave-fill." The morphology and constitution of this cave-fill depend on what materials it is made of. These materials, mostly from outside the karst, are very varied in their nature, their origins and how they are put in place. The materials and the conditions of deposit are closely related to the external environment and to the internal operation of the karst system. Studying them therefore gives valuable information on the history of the Karst.
THE DIFFERENT TYPES OF CAVE-FILL.
Depending on their origins and nature, there are:
- Detritus, miscellaneous rock debris and sediment, transported and accumulated by gravity or decantation.
- Organic deposits, either from the accumulation of organic debris of vegetable, animal or human origin, or from the construction of sediments by living organisms.
- Chemicals, deposited by precipitation of minerals in solution.
"Concretion” belongs to this type of cave-fill, creating many features adorning caves; this will be dealt with more fully below.
Detritus.
Detritus is formed by sediments composed of debris, more or less modified, from the mechanical destruction of rocks.
Origin
The original rocks in sediments may derive from the karst itself, or from its geological and geographical surroundings. When these sediments come from rocks outside the karst area, they can be identified geologically to find where they originated from and their route to their deposit in the cave can be traced. So by studying the geological nature of the constituents of detritus, the environment outside the cave at the time of their deposit and the direction of the flows that transported them can be discovered.
Sizes
The detritus in sediments is very variable in size and weight. We distinguish between:
- Boulders: large angular rocks
- Pebbles: medium-sized, more or less rounded stones.
- Sand: small rounded granules, often of a single mineral.
- Silts and clays: very fine elements.
Deposition - Conditions of deposit.
The shape of the elements and how they are transported are closely interconnected. On the one hand, the size and weight of the elements determine how they are transported and deposited, but on the other hand the duration and conditions of transport alter the shape of the elements transported. As they are carried along, the rocks crash together or roll along channels, are blunted, rounded and reduced in size. Boulders turn into pebbles and sand. The longer the transport is in time and distance, the smaller the elements become. In addition, boulders demand much more energy to transport than sand; it takes a violent, torrential type of flow to move them. Which means that if the energy of the flow reduces, larger elements cannot be moved. They will remain still while the fines particles will continue to be carried along by the slower water. So deposits of large angular blocks reveal transport over a short distance, by landslides or violent currents, whereas sand and clay are evidence of deposits often spread over long distances in calm water or on beaches. So the nature and the location of detritus reflect the strength of water flow and the conditions of circulation of the water at the time they were laid down, whether in caves or outside.
Organic deposits.
These generally derive from outside the caves, but like detritus may also be internal by modification or construction.
Origins
Organic components of deposits have various origins:
- Vegetable: Wood, leaves, pollen,
- Animal: Bones, droppings, but also built up by organisms such as algae or bacteria,
- Human: Bones, droppings, traces of activity (homes, industries, various alterations),
- Secondary: these result from the reworking or decomposition of the above (breccias, phosphorites, lignite, etc.).
Deposition – Value
As for detritus these sediments can be deposited by the flow of water, but they may be directly deposited by living organisms capable of entering the underground environment themselves. By studying these deposits we can:
- Trace the environment (fauna, flora, and climate)
- Trace human history (homes, habitats, culture, etc.)
- Perform carbon dating (carbon 14)
Chemical deposits
These are deposits, by precipitation and crystallisation, of dissolved substances generally transported as liquids.
The operation of the karst creates conditions particularly favourable to this type of deposit, the result of which is "concretion".
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