Environmental Engineering Reference
In-Depth Information
FIG 126. Green Cwm, a good example of a dry valley system, with the Llethrid Stream flowing underground and
emerging at the Parkmill resurgence. (Harold Grenfell)
InnorthGowerasmallsectionoflimestone,whichreceiveswaterdrainingfromCefnBryn,collectsit
in a number of sinks, or swallets, which have their resurgences (the locations where the streams reappear
at ground level) at Leason and Llanrhidian. This is the form associated with classical limestone drainage
patterns. Water passes off an impermeable rock, sinks into the limestone and re-emerges at a scarp-like
slope, in this case represented by the old cliff line of the estuary. Similarly on the south side of Cefn Bryn
there are more sinks in the Nicholaston area, adjacent to the south Gower road. All are visible from the
road, but are situated on farmland. The Nicholaston sinks occur almost immediately the streams leave the
Old Red Sandstone of Cefn Bryn and reach the soluble limestone, and take the form of circular or oblong
depressions, with a hole which takes the water into the ground. Downstream of the sinks shallow dry val-
leys are visible and these were probably the courses of the streams before the development of the more
recent sinks. They meet the old cliff line in deep gullies, which go down the steep cliff face to the sand
dunes and shoreline at Nicholaston Pill. The systems at Nicholaston have, however, been affected by ag-
ricultural improvement and many of them are piped where the stream crosses arable land.
AgoodexampleofadisappearingstreamistheLlethridStream,thevalleyitselfbeingafineexample
of a complete dry valley system extending across the limestone outcrop, between a stream sink and a re-
surgence,eachclosetotheboundaryofthelimestone.Thestream,whichrisesonForestCommon,passes
beneath the north Gower road and enters a swallow-hole about 200 metres to the south at the base of a
wooded slope. This main sink is the entrance to Llethrid Swallet, while in the left bank below the sink
liesToothCave.Theundergroundrouteofthewatercourseisinaccessible,butLlethridSwalletandTooth
Cave have passages that act as a flood overflow route. For a distance of about 2 kilometres from this hole
the stream passes underground, along Green Cwm, to emerge at the Parkmill, or Wellhead, resurgence.
This resurgence is impenetrable as it now lies under a pool impounded to allow the pumped abstraction
of the water. Llethrid Swallet provides about 20 per cent of the water at the resurgence, the rest coming
from percolation input and other smaller sinks. The flow time through from Llethrid Swallet is about 20
hours. On the opposite side of the valley to the resurgence is a smaller spring, Kitchen Well, which is fed
entirely by percolation water. Below the resurgences the stream flows over the surface to reach the sea at
Three Cliff Bay via the Pennard Pill (Figs 127 & 128).
Search WWH ::




Custom Search