Carreg Cadno
Encyclopedia
Carreg Cadno is a hill five miles northeast of Abercraf
in the county of Powys
, south Wales
. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark
. Its summit at OS grid ref SN 874161 reaches a height of 538m / 1763 ft above sea level. The hill is within the Ogof Ffynnon Ddu
national nature reserve
which is owned and managed by the Countryside Council for Wales
.
, a coarse quartzitic sandstone (gritstone), formerly known as the Basal Grit
of the Millstone Grit
Series, and laid down during the Namurian
stage of the Carboniferous period. The northern slopes are formed from the underlying Carboniferous Limestone
. The area displays various features typical of karst
ic landscapes including hundreds of shakeholes in both the limestone and the gritstone areas. Substantial areas of gritstone country have foundered
as the underlying limestone has dissolved away and caverns have collapsed. There are also areas of both limestone pavement
and gritstone pavement. The latter occasionally retain striations on their polished upper surfaces due to scratching by stones embedded in the base of a moving icesheet during the last ice age
some 18,000 years ago.
There are also pockets of silica sand where the gritstone has been intensively weathered. An economically valuable deposit of silica sand was worked near Pwll Byfre for many years and transported by tramroad to make refractory bricks ('firebricks' or 'silica bricks') at the nearby Penwyllt
Dinas Silica Brick Works.
Ogof Ffynnon Ddu
, Britain's deepest cave lies under the northern slopes of Carreg Cadno. The vertical interval between the sink of the Byfre Fechan stream at Pwll Byfre and its resurgence at Ffynnon Ddu is 300m / 1000 ft. The national nature reserve has been established to protect both the surface features and the vegetation and habitats which they provide and also the extensive cave network beneath.
to Penwyllt
. Many walkers start in a car park beside the limestone quarry at Penwyllt
(OS grid ref SN855156).
Abercraf
Abercraf is a village within the historic boundaries of Brecknockshire, Powys, mid Wales.- Location :Abercraf lies in the extreme south of the county, in the Upper Swansea Valley 2.5 miles northeast of the small town of Ystradgynlais...
in the county of Powys
Powys
Powys is a local-government county and preserved county in Wales.-Geography:Powys covers the historic counties of Montgomeryshire and Radnorshire, most of Brecknockshire , and a small part of Denbighshire — an area of 5,179 km², making it the largest county in Wales by land area.It is...
, south Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark
Fforest Fawr Geopark
Fforest Fawr Geopark was the first Geopark to be designated in Wales having gained membership of both the European Geoparks Network and the UNESCO-assisted Global Network of National Geoparks in October 2005. The Geopark aims to promote and support sustainable tourism and other opportunities to...
. Its summit at OS grid ref SN 874161 reaches a height of 538m / 1763 ft above sea level. The hill is within the Ogof Ffynnon Ddu
Ogof Ffynnon Ddu
Ogof Ffynnon Ddu is a cave located under a hillside in the area surrounding Penwyllt in the Upper Swansea Valley in South Wales...
national nature reserve
National Nature Reserve
For details of National nature reserves in the United Kingdom see:*National Nature Reserves in England*National Nature Reserves in Northern Ireland*National Nature Reserves in Scotland*National Nature Reserves in Wales...
which is owned and managed by the Countryside Council for Wales
Countryside Council for Wales
The Countryside Council for Wales is an Assembly Government Sponsored Body. It is the Welsh Assembly Government's wildlife conservation authority for Wales...
.
Geology
The southern slopes of the hill are formed from the Twrch SandstoneMarros Group
The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous period in South Wales. These rocks were formerly known as the Millstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of the Pennines and...
, a coarse quartzitic sandstone (gritstone), formerly known as the Basal Grit
Marros Group
The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous period in South Wales. These rocks were formerly known as the Millstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of the Pennines and...
of the Millstone Grit
Millstone Grit
Millstone Grit is the name given to any of a number of coarse-grained sandstones of Carboniferous age which occur in the Northern England. The name derives from its use in earlier times as a source of millstones for use principally in watermills...
Series, and laid down during the Namurian
Namurian
The Namurian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 326 and 313 Ma . It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Namurian is named for the Belgian city and province of Namur where strata of this age...
stage of the Carboniferous period. The northern slopes are formed from the underlying Carboniferous Limestone
Carboniferous limestone
Carboniferous Limestone is a term used to describe a variety of different types of limestone occurring widely across Great Britain and Ireland which were deposited during the Dinantian epoch of the Carboniferous period. They were formed between 363 and 325 million years ago...
. The area displays various features typical of karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
ic landscapes including hundreds of shakeholes in both the limestone and the gritstone areas. Substantial areas of gritstone country have foundered
Foundered strata
Foundered strata is a term used by the British Geological Survey in its maps and geological memoirs, and indeed by other authors, to describe rock strata which has collapsed due for example to the dissolution of underlying strata....
as the underlying limestone has dissolved away and caverns have collapsed. There are also areas of both limestone pavement
Limestone pavement
A limestone pavement is a natural karst landform consisting of a flat, incised surface of exposed limestone that resembles an artificial pavement. The term is mainly used in the UK where many of these landforms have developed distinctive surface patterning resembling block of paving...
and gritstone pavement. The latter occasionally retain striations on their polished upper surfaces due to scratching by stones embedded in the base of a moving icesheet during the last ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
some 18,000 years ago.
There are also pockets of silica sand where the gritstone has been intensively weathered. An economically valuable deposit of silica sand was worked near Pwll Byfre for many years and transported by tramroad to make refractory bricks ('firebricks' or 'silica bricks') at the nearby Penwyllt
Penwyllt
Penwyllt is a Welsh hamlet located in the upper Swansea Valley, to the east of the Black Mountain.A former quarrying village, quicklime and silica brick production centre, its fortunes rose and fell as a result of the industrial revolution within South Wales...
Dinas Silica Brick Works.
Ogof Ffynnon Ddu
Ogof Ffynnon Ddu
Ogof Ffynnon Ddu is a cave located under a hillside in the area surrounding Penwyllt in the Upper Swansea Valley in South Wales...
, Britain's deepest cave lies under the northern slopes of Carreg Cadno. The vertical interval between the sink of the Byfre Fechan stream at Pwll Byfre and its resurgence at Ffynnon Ddu is 300m / 1000 ft. The national nature reserve has been established to protect both the surface features and the vegetation and habitats which they provide and also the extensive cave network beneath.
Access
The hill is designated as open country and so freely available to walkers. A public footpath followed by the Beacons Way crosses the northern flanks of the hill in an east-west arc from YstradfellteYstradfellte
Ystradfellte is a small village in Powys, Wales, in the Fforest Fawr area of the Brecon Beacons National Park.Because of the geography and topography of the area the village of Ystradfellte is not visible from the nearest main road, the A4059 Hirwaun to A470 stretch.Ystradfellte comes under...
to Penwyllt
Penwyllt
Penwyllt is a Welsh hamlet located in the upper Swansea Valley, to the east of the Black Mountain.A former quarrying village, quicklime and silica brick production centre, its fortunes rose and fell as a result of the industrial revolution within South Wales...
. Many walkers start in a car park beside the limestone quarry at Penwyllt
Penwyllt
Penwyllt is a Welsh hamlet located in the upper Swansea Valley, to the east of the Black Mountain.A former quarrying village, quicklime and silica brick production centre, its fortunes rose and fell as a result of the industrial revolution within South Wales...
(OS grid ref SN855156).