ORIGINS OF CERTAIN CAVE FEATURES
The physical wearing of the limestone by the flowing water is called EROSION. During the formation of the passages at Kents Cavern two different types of erosion took place.
PHREATIC erosion occurred at the beginning of the caves' formation, when the limestone of the present-day hillside was below the WATER TABLE. The water entering the limestone was under pressure. It could move in any direction, even uphill. Cracks, and subsequent passages, were completely filled with water and this allowed erosion to take place over the whole surface, on the walls, the floor and on the ceiling.
VADOSE erosion followed later when the passages had been enlarged, and external land movements had lowered the level of the water table in relation to the limestone hillside. This allowed air to enter the passages for the first time and the flowing water occupied only the lower portion of the growing caves. Consequently this erosion only affected the lower parts of the walls and the floors of the caves.
STRIATIONS in the ceiling of the caves indicate a variation in the direction of flow of the water through the passages. This may possibly have happened during the change from phreatic to vadose erosion.
ROOF PENDANTS (hanging rocks) are caused by adjacent tubes and passages, which are opened up by phreatic erosion, merging together to form a single, larger space. Later, during the vadose stage of development, the lower half of the tubes and passages is removed as the floor is further eroded. The upper half is left 'hanging' from the ceiling.
SCALLOPING, usually found in the ceiling and upper parts of the walls, is an effect caused by irregularities in the course of the waterflow. These irregularities cause an 'eddying' in the waterflow and the resulting erosion produces the patterned effect.
SEDIMENTS IN THE CAVE
When water entered the caves it was never 'clean'. It always carried some form of sediment or dissolved material. The suspended material (that which does not dissolve) often increased the eroding power of the water, but was usually left behind (deposited) as the water percolated through to a lower level.
During the formation of the caves surface material from the hillside above was brought in through vertical passages. Soil, glacial material, river silt, organic materials, animal bones from the prey of animals and the food of early man were all carried into the caves.
Today this material is found in the form of a layer of ancient deposits, known as BRECCIA, lining the floor of the caves. The breccia is poorly stratified (layered) and poorly sorted. Coarse material, often angular in shape, is held in a matrix of the finer material now hardened into a solid state, quite often very hard indeed. It is not possible to date the breccia itself, only some of the material which has been buried within it in the past. It is believed to be over 400,000 years old.
In more recent times sections of the breccia have been eroded from within the caves. Part was removed through lower passages as yet unfound, whilst the rest was redeposited to form the cave earth, which in turn was partially removed during the Victorian excavations at the mouth of the caves.
The dissolved material carried by the water was chemically released to form the characteristic CALCITE features, such as stalactites and stalagmites collectively known as SPELIOTHEMS.
A single drop of water seeping out of a small joint in the limestone will hang for some time on the cave roof. As carbon dioxide is absorbed into the air, a ring of calcite is deposited around the edge of the droplet of water. When the droplet falls to the floor, another takes its place and the process is repeated over and over again. In this way a cylindrical STRAW is formed with water feeding down inside it. Straws have an almost constant diameter because of the uniform size of the water droplets. They are extremely fragile and often break off under their own weight.
If water flows down the outside of a straw, carbon dioxide will be lost into the cave atmosphere and a thicker layer of calcite builds up. This results in a cone-shaped deposit, called a STALACTITE, tapering down from the ceiling. They often grow close together forming STALACTITE CLUSTERS.
Sometimes water may run down a sloping cave roof and deposit a thin streak of calcite. Further waterflow adds to the deposit forming a hanging sheet of calcite known as a CURTAIN. Where curtains display different colours they are often nicknamed 'Rasher of Bacon' or 'The Flitch'. Sometimes they hang from the cave roof in the shape of a SHARK'S FIN.
Irregular crystal growth may produce calcite formations which twist and curve in every direction from the roof, walls and even stalactites. These very beautiful formations are called HELICTITES.
Water dripping from all these formations will fall to the cave floor. When water splashes on the ground it spreads out to form a disc of calcite. As the process continues a STALAGMITE builds up. When a stalagmite grows immediately below a stalactite they may eventually join to form a COLUMN or pillar. Water flowing down cave walls will decorate them with thick layers of calcite known as FLOWSTONE.
Pools of water produce their own type of calcite deposit. CRYSTALS may begin to grow in the pools when they become full of calcium-rich water. In most caves the crystals are pyramid-shaped and line the floor and edges of the pool. Water flowing over the edge of the pool may deposit calcite along its rim to form a barrier called a GOUR DAM. Staircases of gour dams and pools can be seen in many Devon caves.