Environmental Engineering Reference
In-Depth Information
Many topics have been written about the Gower caves, and there is a whole publication devoted to
every aspect of the most famous Gower cave, Goat's Hole, Paviland (Aldhouse-Green, 2000). As de-
scribedinChapter4,thelatter contained the'RedLady',theonlyUpperPalaeolithic ceremonial burialin
theBritish Isles.Theemphasis ofallthese publications, however,iseither onarchaeology andprehistoric
animal remains or on recreational caving. In contrast to the vast amount of information that is available
on the previous occupants of the caves, little is known about the wildlife present today.
THE ORIGIN OF THE CAVES
The present coastal caves are the remnants of once extensive systems, some parts of which have been
removed by coastal erosion and some filled by debris from the last ice age. Generally glacial periods are
times of cave infilling and degradation with many passages becoming completely choked by deposits.
When the caves were first used by people the passages emerged from a cliffline south of the present-day
coast and they would have overlooked wide coastal plains with the sea lower and more distant than at
presentPrimitive cavepassages,formedbythelimestone beingdissolvedbyfreshwater,almost certainly
existed before the sea exposed them, but it is difficult to distinguish caves formed by the sea, and later
modified by the penetration of fresh water, from existing passages altered by the erosion of the coast.
A number of caves open in the coastal cliffs, however, such as Bacon Hole and Minchin Hole, are al-
most certainly very old solution-cave fragments, these two being now most notable for their extensive
sequences of Pleistocene sediments and archaeological material. The inland caves such as those in Green
Cwm owe their origins to groundwater penetrating suitable geological strata.
Although water cannot penetrate solid limestone it is able to flow through cracks in the rocks. It is
these cracks that are the key to understanding the origin of most of the Gower caves. Caverns in the rock
form principally by means of a simple chemical reaction in which hydrogen ions from groundwater, acid-
ified with dissolved carbon dioxide, act on the relatively insoluble carbonate ions in the limestone to pro-
duce soluble bicarbonate ions, which are then washed away. It has been estimated that the process makes
the limestone 25 times more soluble than it would be in pure water, and the result is holes in the rock.
This first stage of cave formation, known as 'initiation', creates the openings through the rock that
permit the flow of groundwater. The main stage of cave development, however, is 'enlargement', when
thesmallinitialfissuresareenlarged.Thisisfollowedby'degradation',whichistheterminalphasewhere
the cave either collapses, is filled with sediment, or is removed by a lowering of the overlying surface. In
a complex cave system all three processes take place simultaneously in passages at different depths and
positions in the limestone. Enlargement and sediment infilling can also take place at the same time in a
single passage. The timescales for cave formation are long and the initiation phase can take tens of mil-
lions of years while the enlargement of a cave passage to a metre in diameter can take up to ten thousand
years.
While cave formation is determined totally by the geology, the flow of water starts to influence the
development of the cave soon after the initial openings are created. As the limestone fissures are enlarged
the permeability of the rock increases and the fissures and caves are drained so that they contain air-filled
spaces and a water table is established. Below the water table the openings remain submerged and the
cavethereforedevelopsdifferentlyaboveandbelowthewaterlevel.Inparticular thewaterflowbecomes
more turbulent and there is increased erosion due to mechanical abrasion by sediment from the surface.
As the cave system evolves it increases in complexity as individual passages are abandoned in favour of
new routes at lower levels. Finally degradation and destruction of cave passages involve filling and chok-
ing by sediments, collapse and complete removal, in most cases after the cave has been abandoned by its
formative stream.
