The earth-cache is situated in ‘Waterfall Country’ in a
triangle formed by the villages of Hirwaun, Ystradfellte, and
Pontneddfechan. Waterfall Country includes the head waters of the
rivers Nedd, Mellte, Pyrddin and Hepste. These rivers drain the old
red sandstone mountain ranges of the Fan’s in the north and
travel south through the deep V shaped wooded valleys the rivers
have formed in the carboniferous limestone of the southern belt of
the national park.
These interlinking complex systems of rock formation are the
bedrock of the landscape we know as the Brecon Beacons National
Park. Over geological time various factors and forces served to
shape this underlying bedrock into the landscape we see.
The central feature of the Brecon Beacons are the mountain
ranges that extend across the full width of the National Park. The
National Park is divided into three distinct collections of
mountain escarpments named the Black Mountains (eastern side); the
Brecon Beacons (the central area) and sometimes confusingly in the
singular the Black Mountain or Carmarthen Fan (Western area).
The geology of most of the Brecon Beacons National Park consists of
old Red Sandstone a sedimentary rock laid down more than 300
million years ago. Confusingly, Old Red Sandstone is not always
red, though the underlying presence of its rich reddish hue is
often a backcloth beneath the green vegetation in more than two
thirds of the 519 sq miles of the National Parks area.
In this old red sandstone geological system has a number of
subdivisions including red marls (a deep rich red colour) , Senni
beds (greenish hue), Brownstones, Plateau beds, and Grey Grits. Old
Red Sandstone may therefore be thought of as a generic term.
The brownstones are the building blocks of the main mountain
ridges. However, the distinctive billiard table summits of mountain
peaks in the Brecon Beacons such as Pen Y Fan, Corn Ddu Fan Hir and
Fan Gyhirych are topped by "plateau beds". Plateau beds are tough
gritty sandstones with occasional quartz pebbles and form a
resilient capping
In the southern half of the Brecon Beacons National Park, due
south of the small village of Ystradfellte, there lies an area of
Carboniferous Limestone and Millstone Grit containing steep sided
gorges, cave systems and numerous spectacular waterfalls.
In stark contrast to the old red sandstone geological system the
southern zone of the national Park consists of a Carboniferous
limestone system that extends from the far west to the southernmost
tip in the east. Limestone country is easy to recognize it is
consistently grey in stark contrast to the reds and browns of the
predominant old red sandstone.
This southern edge of the Brecon Beacons National Park the belt of
carboniferous limestone wedged between Old Red Sandstone to the
north and the Millstone Grit containing coal deposits to the
south.
One of the characteristics of carboniferous limestone is that it
is susceptible to the action of slightly acidic water. Rain falling
in this locality absorbs carbon monoxide from the air thus creating
a mild solution of carbonic acid. As this slightly acidic water
passes through the naturally occurring cracks and fissures in the
carboniferous limestone over geological time it has created the
extensive cave systems found along this southern edge of the Brecon
Beacons National Park.
The coal measures of such significance in the history of South
Wales and one of the building blocks of the Industrial Revolution,
are only found in a few small areas within the National Park.
Within the carboniferous system on the southern edge of the
National Park can be found Millstone Grit. Sometimes referred to as
Farewell Rock so called because once a coal miner encountered this
rock no coal is found worth working beyond this strata. It is
popular with climbers and has been used for the hearths of iron
furnaces being resistant to intense heat.
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.
Waterfalls occur at places in the river where the topography or
height changes. Water runs over a ledge and falls usually
vertically to the next level of the river. The ledges are usually
made of a more resistant bedrock layer, different from the rest of
the river's parent material. The water running over the ledge cuts
into the rock under the ledge producing the falls and grade to the
waterfall. Eventually the erosional forces will undermine the ledge
and cause it the break off. This is what causes the upstream
retreat of a waterfall. As this action continues, eventually after
many years, the waterfall will disappear leaving only rapids as the
trace it ever existed.
In this valley there are 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, and ultimately
causing them to collapse.
Often, the rock stratum just below the more resistant shelf will
be of a softer type, meaning that undercutting due to splashback
will occur here to form a shallow cave-like formation known as a
rock shelter or plunge pool under and behind the waterfall.
Eventually, the outcropping, more resistant cap rock will collapse
under pressure to add blocks of rock to the base of the waterfall.
These blocks of rock are then broken down into smaller boulders by
attrition as they collide with each other.
Streams become wider and shallower just above waterfalls due to
flowing over the rock shelf, and there is usually a deep pool just
below the waterfall because of the kinetic energy of the water
hitting the bottom. Scwd yr Eira is one of the few falls where
people can walk behind the water as it cascades down into a
pool.
Although Scwd yr Eira Waterfall attracts 160,000 visitors a year
it is an exceptionally hazardous walk. There are many approaches,
as this is ‘classic walk’ of about 4 miles. Good
walking boots are advisable as the path can be extremely
treacherous in parts.
To claim this earth cache.
Email me the answers to the following questions BEFORE logging.
Don’t include the answers in your log even in an encrypted
form.
1. BEFORE you proceed down the steps take a picture with you and
/ or your GPS with one of the hazard signs there. Pay CLOSE
attention to this sign!!!!
2. Describe the weather conditions before and during your
visit.
3. Estimate the size of the ‘rock shelter’ height /
width / depth
4. Describe rocks the left hand side as you face the falls don't
worry about trying to identify them.
5. THEN upload a picture and log the earth cache.
6. For extra merit for post a picture with people actually behind
the falls. Risk assess FIRST!!!!!
