|Regions \ Wales|
Most of the natural caves in Wales are to be found in the South where the limestone outcrops around the fringes of the South Wales Coalfield. There are, however, some significant caves to be found in North East Wales.
The lengthy major systems of the South are formed in the Northern Outcrop of the limestone that runs roughly from Ammanford in the West to Abergavenny in the East, parallel to, or along the Heads of the Valleys Road.
Traditionally, and by convention, newly discovered caves are given names in the Welsh Language rather than English.
Natural caves are found in those areas where Carboniferous Limestone occurs in south and in northeast Wales. In the south it outcrops as a thin band around the margins of the South Wales Coalfield, an area which was a major depositional basin during most of the Carboniferous Period between 359 and 299 million years ago. Though the limestone is present beneath the coalfield, cave development has taken place only within the zone of groundwater circulation around its margins. The most important of these zones is the 'north crop' between Ammanford and Abergavenny. This same depositional basin extends westwards through Gower and beneath Carmarthen Bay to south Pembrokeshire. Cave development has been limited further west.
A separate depositional basin centred on the Forest of Dean and extending down the Wye valley is also an important caving area. The southern part of the basin is continuous at depth beneath the Severn estuary with that in the Bristol area.
A discontinuous band of limestone wraps around the northeastern margin of the Welsh massif from south of Oswestry northwards into Flintshire. It dips eastwards towards the Shropshire and Cheshire Plains and is heavily faulted. A further band in Denbighshire runs from near Ruthin to the coast at Abergele and then westward via Great Orme to central Anglesey and the Menai Strait.
In general terms, stratigraphy is the science of dividing up the succession of rocks in an area into definable layers ('rock units') with particular characteristics, giving them names and, where possible, determining their ages. To complicate matters, the names that geologists have used have changed over the years. A hierarchy of terms is used, including supergroup, group, subgroup, formation, member and bed.
Stratigraphy of South Wales
The details of the various rock units which together make up the Carboniferous Limestone of South Wales differ from place to place but in general the sequence can be divided into a lower ie older, 'Avon Group' and an upper, and therefore younger, 'Pembroke Limestone Group'. The Avon Limestone Group is what used to be called the 'Lower Limestone Shale Group' whilst the 'Pembroke Limestone Group' replaces the old 'Main Limestone' and (as the Oystermouth Formation) the old 'Upper Limestone Shale Group'.
Carboniferous Limestone Supergroup
- Pembroke Limestone Group
- Oystermouth Formation
- Oxwich Head Limestone Formation
- Dowlais Limestone Formation
- Llanelly Formation
- Abercriban Oolite Subgroup (in west) / Clydach Valley Subgroup (in east)
- Avon Group
- Cwmyniscoy Mudstone Formation
- Castell Coch Limestone Formation
- Tongwynlais Formation
The Oxwich Head Limestone includes two distinctive thinner rock units within it; the 'Honeycombed Sandstone Member' and the 'Penderyn Oolite Member'. Where seen at outcrop the Honeycombed Sandstone has a characteristic honeycombed appearance. It is not continuous across the area ie is not always present in the sequence.
Stratigraphy of North Wales
The details vary across the area but broadly encompass the following formations overlying various 'basement beds':
- Clwyd Limestone Group
- Minera Formation
- Cefn Mawr Limestone Formation
- Loggerheads Limestone Formation
- Leete Limestone Formation
- Llanarmon Limestone Formation
- Black Mountain
- Swansea Valley
- Neath Valley
- Taff Valley
- Llangynidr Mountain
- Llangattock/Clydach Gorge
- Southern Outcrop
- North East Wales
- Gwydyr Lead Mines
- Great Orme Copper Mines
- Lleyn Peninsula Manganese Mines
- Meirionydd Gold Mines
- Meirionydd Manganese Mines
- Meirionydd Slate Mines
- Mid Wales Lead Mines
- North East Wales Lead Mines
- Other Slate Mines
- Parys Mountain
- Snowdonia Mines
- South Wales Metal Mines
- Big Pit Mining Museum, Blaenavon
- Chwarel Hen Llanfair
- Dolaucothi Gold Mine
- Great Orme Bronze Age Mines
- Llechwedd Slate Caverns
- Llywernog Lead Mine
- Sygun Copper Mine
- Cambrian cave registry - locations and details of most cave sites in Wales and the Marches. Hosted by the Cambrian Caving Council.