Devonian Limestone formed from the debris of coral reefs, several degrees South of the Equator, 370 million years ago. The 'plate' of the Earth's surface on which we now 'sit' slowly moved northwards to its present position. Around 220 million years ago the great forces involved in the formation of the granite deposits of Dartmoor, known as the Hercynian Orogeny, caused the limestone block in which Kents Cavern is today situated to be fractured and folded. At this time the limestone was still buried beneath the sea. It remained buried and was not 'uplifted' to its present position until the beginning of the Tertiary period, some 50 million years ago. Uplifting has occurred on an irregular basis ever since, and is still occurring today. This can be illustrated by the raised beaches in the area, namely the older they are, the higher they are.
Through the fractures in the limestone rainwater and streamflow filtered down to the water table. Horizontal movement was also possible along the bedding planes of the
limestone, the boundaries between two successive layers of limestone deposited one on top of the other. The rainwater entering the limestone was slightly acidic, due to a chemical reaction with the carbon dioxide in the atmosphere, and became further concentrated after its passage through the humus layer (decaying plant material) in the soil.
Water + Carbon Dioxide = Carbonic Acid ( H20 + C02 = H2C03 )
A chemical reaction then takes place within the limestone;
Acid + Limestone = Calcium Bicarbonate ( H2C03 + CaC03 - Ca(HC03)2 )
This is how the limestone was removed from the initial cracks and fractures. Small passages were eventually cut through the rock. With the water table still quite high (very near to the top of the present-day hillside), these passages were completely filled with water (Phreatic tubes). The remains of these tubes can be traced in the walls and ceilings of the present caves. With the aid of another cave feature known as a 'Scallop', a small impression eroded out of the surface of the rock by water turbulence, the direction of flow of the water through these tubes can be identified.
These features show that the water originally entered the limestone in the region of the High Level Chamber at the back of the Inscription Chamber and through the wall on the lefthand side of the Rocky Chamber (Facing into the Chamber). The water flowed from these points towards the outlets situated at the present Main Entrance, in the lefthand corner at the back of the Bears Den and in the northeast gallery ( the corner of the Vestibule opposite to the Main Entrance ). Erosion and chemical dissolution took place around the whole circumference of the tube and it grew accordingly.
During this period much of the debris caused by the expansion of the passages was deposited on the floor of the caves. Water continued to flow over the top of the deposits and caused upward expansion of the passages by further erosion and dissolution of the ceiling. For example, the 'pockets' in the roof of the Long Arcade were formed upwards into the ceiling of a developing passage, probably around present-day head-height, which was full of stream sediment.
Then the water table dropped to a lower level, due to deepening of the valley outside. The passages became air-filled with streams flowing across the floor. Consequently further expansion (due to continued erosion and dissolution) was restricted to the lower portions. During this period the caves reached their maximum size.
A feature within the caves from this period is the notch carved out of the wall of the Long Arcade (Halfway along on right hand side going in). The 'floor' of this notch (about knee height) is virtually horizontal. A whole section of rock was carved out by a stream flowing at the level of the notch.
After the water table had fallen by about 6 metres a 'new' outlet at the rear of the Wolf's Den allowed water to flow under the Vestibule, probably to emerge somewhere down the slope in front of the caves (probably now submerged by development). Another course took water through the Charcoal cave and out through the Sally Ports, exits which have also been covered over by excavation spoil and development.
The water table subsequently dropped a second time, leaving the caves high and dry.
Somewhere around half a million years ago parts of the roof collapsed, opening the caves to the outside and allowing a thick, semi-liquid deposit to enter the passages. This entry might have taken several thousand years (at present we have no definite idea). The 'sludge' drained and subsequently formed the Breccia deposits which are causing such great interest at the moment.
It is thought that the roof collapsed in at least three places, namely at the back of the Bears Den, in the High Level Chamber at the back of the Inscription Chamber and at the Great Oven, halfway between the Bears Den and the Inscription Chamber. Evidence of this roof collapse can be seen in the Bears Den, where the large boulders on the floor at the back of the cavern have been identified as having come from the roof. These boulders today form the bottom part of the 'wall' retaining the spoil removed from beneath the Bears Skull.
The entry of the 'sludge' was probably not continuous as very thin layers of 'ponding' deposits, in the form of a fine clay-like material, can be detected dividing up the previously defined unstratified mass of the Breccia. These 'ponding' deposits were probably formed as water drained from the Breccia debris flow during quieter periods.
Bears are known to have been using the caves as hibernating and breeding chambers throughout this period. Man however was almost definitely not using the passages, merely sheltering in the mouths of the caves for short periods. Evidence of human activity, in the form of crude hand-axes fashioned from flint, were discovered by William Pengelly in the 1870's. Today they are assumed to have been dropped outside on the hilltop and subsequently washed into the caves as part of the 'sludge'. Virtually all of the 'finds' were made in the material 'arriving' via the High Level Chamber. Pengelly did not recognise this fact and thus he failed to differentiate between the various breccia deposits defined today.
Because the bear remains were buried during the deposition of the breccia their age is contemporary with that of the deposits. However, as the human artefacts are assumed to have washed into the caves from the hillside above, their age could be considerably greater than that of the breccia. Further study is needed before we can solve this dilemma and unfortunately present techniques do not go far enough to permit a definite answer in the near future.
Eventually the Breccia built up to such a level that it closed off the very entry points through which it arrived in the first place. There followed thousands of years of calm within the passages, during which the crystalline stalagmite floor, which can today be seen in the Bears Den, over the Bridge, in the Inscription Chamber and in the Rockies was laid down. This deposition started c.350.000 years ago and continued for 1/4 million years. In some places, such as Hedges Boss, it is continuing to this day. Stalagmite samples have recently been scientifically dated and results appear to agree with this theory, as calculated ages range from 100 up to 350+ thousand years.
We know little of these times (100 - 350 thousand BP) at present, although from sites elsewhere in Britain we do know that they cover at least 3 different interglacial periods (Hoxnian, Aveley & Ipswichian) and intervening glacial periods. Stalagmite was deposited in all but the coldest times, although the rate of deposition would vary greatly depending on the outside climate. For instance, in warmer conditions the concentrations of minerals carried by the incoming water would be much higher. These higher concentrations would occur because the water was at its maximum acidity due to it having passed through a consider layer of rotting plant material in the soil (humus; the humus layer having built up due to the good growing conditions for plants during the warmer spell of weather.
Once more the caves were opened to the outside. On this occasion the openings were probably the very same ones used today, namely the North and South entrances to Kents Cavern, and also possibly the North Sally Port, subsequently closed during Victorian excavations. Through these openings the breccia and stalagmite layers were broken and eroded by flooding, frost damage and roof collapse. This can be seen in the ceiling of the Great Chamber, where due to the permanent nature of the opening to the outside, temperature fluctuations caused serious roof collapse.
Debris was washed into the caves from the surrounding hillsides burying large chunks of the older material, forming the Cave Earth deposits identified by Pengelly and further excavated at the back of the Wolf's Den during 1991. These deposits have built up over the last 100 thousand years and were capped by a second stalagmite 'floor' (granular layer) at the end of the last glacial period, c. 10,000 years ago.
Beneath this calcite layer Pengelly discovered a discoloured region he very appropriately named the Black Band. The reason for the coloration was the high incidence of ground charcoal within the top layer of cave earth. This definitely put Man inside the caves before the end of the last Ice Age, perhaps 15,000 thousand years ago. On top of the calcite layer there were much greater amounts of charcoal, in what Pengelly defined as the Black Mould, indicating a far greater human usage during more recent times. There has also been identified the largest collection of Late Iron Age pottery in Devon, along with Early Iron Age and Bronze Age pottery and numerous other artefacts from the gradually developing technology of the human visitors over the last 10,000 years.