Environmental Engineering Reference
In-Depth Information
Calcite deposition is common in caves where saturated percolation water issues from fissures, loses
carbon dioxide to the cave air and deposits calcium carbonate to regain equilibrium. Or, as Isaac Hamon
put it in respect of Bacon Hole, 'they say is a drop of water in the top or Rooffe thereof continually, by
wch dropping there is a stone congealed.' Deposition on the cave floor creates tall stalagmites or rounded
bosses, the profiles of which relate to saturation levels and drip rates. Formations such as stalactites and
stalagmites are of considerable scientific value since caves are very long-lived landforms that preserve
valuable evidence of past events on the earth's surface.
Wall and roof collapse is a widespread feature in caves; it modifies the profiles of passages and con-
tributes to cave enlargement where fallen blocks expose new surfaces to attack by water. Major collapses
ultimately block cave passages because the fallen material occupies a larger volume than the undisturbed
rock. A related feature that occurs in the Bishopston Valley is the so-called 'Daw Pits' (Fig. 157). The
word 'daw' is an old name for the jackdaw and the pits were once the haunt of nesting birds, though even
by the 1930s this was only a distant memory. There are a number of such pits including Higher Daw Pit,
Lower Daw Pit, Gulver Pit and Gwynspark Pit. These pits, which appear to be the result of a dramatic
collapseofacavechamber,areanunusualfeatureinlimestoneandoccurasvertical-sidedshaftssome24
metres deep and up to 30 metres in diameter. Gwynspark Pit is the deepest in the valley, being 30 to 35
metres deep with a boulder-strewn floor and steep sides. The collapse of these chambers is considered to
be a relatively recent event, but it is not entirely clear how they were formed, and other suggested causes
includethecoalescingofdrippocketsformedunderanimpermeablecaprockandthesubsequentcollapse
of the cap. Further down the valley is Guzzle Hole, a partially abandoned resurgence where the Bishop-
ston Pill can be seen flowing underground.
