Carboniferous Limestone
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Carboniferous Limestone is a collective term for the succession of
Depositional basins
Within Great Britain the suite of rocks known traditionally as the Carboniferous Limestone Series was deposited as marine sediments in three distinct ‘provinces’ separated by contemporary landmasses. One of these landmasses was the
- Pembroke Limestone Group (uppermost/youngest)
- Oystermouth Formation
- Oxwich Head Limestone Formation
- Penderyn Oolite Member
- Honeycombed Sandstone Member
- Dowlais Limestone Formation
- Abercriban Oolite Subgroup & Clydach Valley Subgroup
- Avon Group
- Cwmyniscoy Mudstone Formation
- Castell Coch Limestone Formation (lowermost/oldest)
The limestone found north of the Wales-London-Brabant Massif and south of the emergent Southern Uplands block is identified as a separate northern province. It is characterised by the presence of numerous ‘blocks’ and ‘basins’ each with its own particular depositional style.
To the north of the Southern Uplands are the limestones of the Scottish Midland Valley stretching from Ayrshire and Arran in the west to Fife, Lothian and Berwickshire in the east. Though of Carboniferous age, the limestones of this Scottish province are not assigned to the Carboniferous Limestone Supergroup.[1]
The Carboniferous Limestone is widespread throughout Ireland.
Geographical extent
The Carboniferous Limestone is a significant landscape-forming rock unit in each of the depositional provinces of Great Britain within which it is found.
South Wales and Bristol area
Within Pembrokeshire the Carboniferous Limestone forms the spectacular coastal cliffs at
An important outlier is that of the Forest of Dean basin which forms the cliffs of the Wye Valley, straddling the England/Wales border and extends southwestwards through Chepstow to Undy.
The larger part of the
The Northern Province
There are limited outcrops on the
The
To the north the limestone is exposed once again in east
North again, it is a major landscape forming feature in the North Pennines and thence through Northumberland to the Northumberland Coast where it extends to the Scottish border at Berwick-upon-Tweed. There are scattered outcrops along the north coast of the Solway Firth.
Midland Valley of Scotland
Limestones occur in southern Ayrshire and in a very broken band running northeastwards through the Pentland Hills towards Edinburgh. There are limited outcrops on the coasts of East Lothian and Berwickshire, isolated outcrops in Fife and Stirlingshire and further occurrences around Greenock and Dumbarton.[14]
Ireland
Carboniferous Limestone occurs most famously around The Burren in County Clare, western Ireland where it produces one of western Europe's most important karst landscapes.
Characteristics
Carboniferous Limestone is a hard sedimentary rock made largely of calcium carbonate. It is generally light-grey in colour. It was formed in warm, shallow tropical seas teeming with life. The rock is made up of the shells and hard parts of millions of sea creatures, some up to 30 cm in length, encased in carbonate mud. Fossil corals,
Carboniferous Limestone has horizontal layers (beds) with bedding planes, and vertical joints. These joints are weaknesses in the rock, which are exploited by agents of both
Surface features
The 'classic limestone walk' is a circular 10 km route from the field centre on the north side of Malham Tarn to the village of Malham, UK via Watlowes Valley and back again via Gordale Scar.
Surface depressions, typically funnel-shaped and variously known as shakeholes, sinkholes, solution hollows and dolines are very common in the Yorkshire Dales and Brecon Beacons. Typically from 1–20 m deep and 1–60 m across, they form as a result of the subsurface collapse of limestone or through the more gradual dropping of surface material into caves.
Streams flowing from higher impermeable slopes sink into the ground when they reach permeable limestone. During dry spells all water sinks very quickly on reaching the limestone, through
Dry valleys are valleys without streams. Watlowes Valley is an excellent example. It was formed originally by a subglacial meltwater stream which existed during the last major Ice Age. After the ice retreated, the valley was further developed by a meltwater stream flowing across the limestone while it was frozen solid. Watlowes Valley is a particularly good example of a dry valley because it has a textbook profile - the south-facing side is less steep than the north-facing side. This results from the weathering and mass movement processes that have operated in the post-glacial period.
A
A gorge is a steep-sided valley, often formed in a limestone area as the result of the collapse of a roof above a cave system. Gordale Scar is an excellent example.
Subsurface features
Caves are common subsurface features in limestone landscapes. In the Yorkshire Dales, there are numerous caves, three of which – Ingleborough Cave, White Scar Caves and Stump Cross Caverns – are now show caves for the public.
In Ireland there is a large number of show caves open to visitors -
The stalagmite and stalactite are the two main subsurface features in a Carboniferous Limestone area. These are formed when rainwater - a weak carbonic acid capable of dissolving limestone - percolates through it via the grykes and joints underground. This means the limestone is pervious. As this happens the limestone is dissolved and removed in solution. Caverns are often found below the surface in the limestone and as the lime-rich water finds its way underground it begins to drip from the roof of the cavern. It is cold underground so there is little evaporation but some does take place leaving a trace of limestone on the roof. Over thousands of years a stalactite forms from the ceiling as the water continues to drip. When the water drips on to the floor of the cavern some evaporation occurs here also leaving a trace of limestone. Again over thousands of years a stalagmite is formed. A stone pillar is formed when a stalagmite and stalactite meet.
Economics
Because it is brittle, the use of Carboniferous Limestone for building stone tends to be limited to those areas where it is the most abundantly available rock. However it is extensively quarried for other purposes:
- It is crushed for roadstone and aggregatewherever it outcrops, particularly in the Mendips and north Wales.
- It is burned for lime in many places. In certain places (e.g. Tunstead in the Peak District, and Horton in Ribblesdalein the Pennines), it is sufficiently pure for production of chemical-grade lime.
- It is used in cement manufacture at plants in England, Wales, Scotland and Ireland.
- In ground form, it is used for power industry flue-gas desulphurisation.
- In many places it is metalliferous, and has yielded lead (in the Peak District and Weardale), and copper (in North Wales, where important Bronze Age mines are to be found).
- It was important in the early Coal Measures in the ironindustry.
See also
References
- ^ Cossey P.J. & Adams A.E. (2004). "British Lower Carboniferous Stratigraphy" (PDF). Geological Conservation Review Series. 29: 1–12.
- ^ British Geological Survey 1:50,000 scale map series sheet E244/5 Pembroke & Linney Head
- ^ British Geological Survey 1:50,000 scale map series sheet E229 Carmarthen, 230 Ammanford & 231 Merthyr Tydfil
- ^ British Geological Survey 1:50,000 scale map series sheet E232 Abergavenny, 249 Newport
- ^ British Geological Survey 1:50,000 scale map series sheets E246 Worms Head & 247 Swansea
- ^ British Geological Survey 1:50,000 scale map series sheet E261/2 Bridgend
- ^ British Geological Survey 1:50,000 scale special map sheet Isle of Man
- ^ British Geological Survey 1:50,000 scale special map sheet Anglesey
- ^ British Geological Survey 1:50,000 scale maps E94 Llandudno, E95 Rhyl, E96 Liverpool, E107 Denbigh, E108 Flint, E121 Wrexham, E137 Oswestry
- ^ British Geological Survey 1:50,000 scale map E99 Chapel en le Frith, E111 Buxton, E124 Ashbourne
- ^ British Geological Survey 1:50,000 scale map E40 Kirkby Stephen, E50 Hawes, E60 Settle
- ^ British Geological Survey 1:50,000 scale map E48 Ulverston
- ^ British Geological Survey 1:50,000 scale map E28 Whitehaven
- ^ British Geological Survey 1:625,000 scale geological map Bedrock Geology UK North NERC 2007