Carboniferous Limestone
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 Wales-London-Brabant Massif, an east–west aligned belt of land stretching through central Wales and the English Midlands to East Anglia and on into Belgium.
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.
Within Pembrokeshire the Carboniferous Limestone forms the spectacular coastal cliffs at St Govan's Head along from which are features such as Huntsman's Leap and the Green Bridge of Wales, a natural arch.
[2] A narrow, intensely quarried outcrop runs inland from Carmarthen Bay through Carmarthenshire from Kidwelly, entering the Brecon Beacons National Park at Llandyfan and extending westwards through the Black Mountain to Cribarth above the upper Swansea Valley.
[3] It then runs near the southern margin of the national park via Trefil and the Llangattock escarpment to Blorenge where it turns southwards.
[6] 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.
There are limited outcrops on the Isle of Man[7] and more extensive ones in Anglesey notably along the Menai Strait, around Benllech and towards Puffin Island.
[11] There are numerous limestone hills in the Arnside and Silverdale AONB and in the southern Lake District e.g. Whitbarrow Scar with coastal exposures around the northern margins of Morecambe Bay such as Humphrey Head.
[12] An outcrop extends from Kirkby Stephen along the western side of the Vale of Eden and wraps around the northern margin of the Lake District as far as Cleator Moor.
Limestones occur in southern Ayrshire and in a very broken band running northeastwards through the Pentland Hills towards Edinburgh.
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.
The Carboniferous Limestone has been folded and faulted by massive Earth movements which can be seen by the fact that the rocks are now above sea-level and no longer horizontal.
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.
In wetter conditions water flows a greater distance across the limestone as underground channels and chambers fill up.
Existing joints are subsequently exploited by the action of chemical weathering carbonation to form deep grykes and rounded blocks called clints.
These are formed when rainwater - a weak carbonic acid capable of dissolving limestone - percolates through it via the grykes and joints underground.
