The Formation of Neath Waterfalls
In South Wales, and the Vale of Neath in particular, the typical
cause of waterfall formation is due to differential erosion of soft
and hard rock strata found together in the river bed.
Where historic movements in the earth's crust has caused
faulting, or glacial erosion has exposed the softer underlying rock
strata to the erosive forces of the river, it will cut more deeply
and quickly into the softer rock creating a drop in the water level
and, consequently, boosting the erosive power of the water.
In the Vale of Neath we find extremely hard, erosion resistant
rocks such a millstone grit and old red sandstone overlying bands
of softer fissured carboniferous limestone and fossiliferous
shales. When the water finds its way through the joints and along
the bedding planes of the harder rock exposing the shales and
limestone, the water soon begins to take its toll on the softer
rocks and so depriving the harder rocks of support, causing them to
collapse - see diagrams.
Whilst in many cases this process causes the formation of
waterfalls when the underlying rock is predominantly limestone, it
is often possible for the whole river to disappear underground
through enlarged fissures in the rock to form caverns. This process
then leaves a dry river bed along its original course which in time
becomes re-vegetated.
The Nedd Fechan and Hepste both disappear for short stages, but
the most dramatic example is where the River Mellte pours
underground for nearly a quarter of a mile to form an extensive
cave system just south of the village of Ystradfellte.
The Porth yr Ogof (portal of the cave) system should not be
entered without proper caving equipment and supervision.
Page Details
Last Updated: 25.05.2010 at 08:42