school energy&environment

Physical deposits

Physical deposits include a large variety of materials that accumulate in caves thanks to gravity (deposits, blocks abd boulders due to break- down) or transported by water. Sediments may be autochthonous, produced in the cave (as in the case of blocks and boulders due to breaking down, or the clay formed by the insoluble minerals in the calcareous rock), or allochthonous, transported into the caves by different agents, generally by water. The material transported by water can be distinguished because its rounded characteristics, the more rounded the grains, the longer is the time they have been being transported and the softer the type of rock. Water capacity to transport material or competence, depends on its energy, in particular its speed and its density and, naturally, the density and weight of the material to be transported. Since most rocks have a density around 2.7 g/cm3, which is therefore the same for all types of rock, this can be considered a constant parameter. Instead of the weight, the mean diameter of the granules or pebbles of material to be transported can be used, i.e. what is called the granulometry. The higher the speed of water, the greater the size of the particles the current is able to transport. The size of the particles vary from millimetric to metric, as for example in large floods where water is dense with sediments. In a cave, the transport of large-sized materials is rare, because even if water were to have a competence able to move large blocks, transportation would soon be stopped by the size of the conduits. The most commonly transported materials are generally sand and pebbles. When the speed of the current decreases, also the competence decreases and water abandons the coarser material, thus carrying out a granulometric classification of the material, i.e. a separation depending on the size of the particles. If coarse material is found in a flooded conduit this means that the current may have high speeds. Since the water is constantly moving, and with very variable speeds, it is not rare to observe a continuous reorganization and changing in the shapes and granulometry of the deposits on the bottom of a gallery. A visit to a well known gallery immediately after a flooding event can hide some surprises that are not always welcome, as for example the occlusion of the narrow and smaller passages, which must be opened again with difficult digging operations, or the presence, mainly near sinkholes, of material carried by the flow into the cave, such as large tree trunks or vegetable rubbish. These changes in the deposits can also cause barrages in the outflow of the vadose zone, thus giving origin to the formation of lakes or siphons. The presence of fine-grained silts and clays on the ceiling and on the walls of a sub-aerial cavity, specially where the material seems humid and fresh, not dusty or dry, can indicate that the gallery may undergo a total flooding, and therefore great care must be paid while exploring during particularly rainy periods. Physical deposits, specially those coming from outside, can provide precious information with regard to the evolution in the region. In fact these may contain remains of rock formations that are now completely worn out by erosion, or testify the passage of glaciers, or document the alternation of hot and cold periods. A detailed study of the chemical and physical deposits in a cave is fundamentally important for the reconstruction of the most recent geological and climatic history.