Carboniferous Limestone

Carboniferous Limestone is a collective term for the succession of limestones occurring widely throughout Great Britain and Ireland that were deposited during the Dinantian Epoch of the Carboniferous Period. These rocks formed between 363 and 325 million years ago. Within England and Wales, the entire limestone succession, which includes subordinate mudstones and some thin sandstones, is known as the Carboniferous Limestone Supergroup.

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. The limestones deposited to its south form a distinct South Wales-Mendip province which extends from Pembrokeshire in the west through southern Carmarthenshire, Glamorgan and south Powys to Monmouthshire, Gloucestershire and north Somerset. These rocks continue eastwards at depth beneath Oxfordshire. The Carboniferous Limestone sequence of South Wales and the Bristol area is currently (2012) subdivided thus:

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.

The Carboniferous Limestone is widespread throughout Ireland.

The Carboniferous Limestone is a significant landscape-forming rock unit in each of the depositional provinces of Great Britain within which it is found.

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. It forms prominent headlands such as those of Stackpole Head and Lydstep Point and the cliffs at Tenby. 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. It is here referred to as the ‘north crop’ as distinct from a sub-parallel outcrop, the ‘south crop’ which defines the southern rim of the South Wales Coalfield. The outcrop continues through Ystradfellte to Pontneddfechan, Penderyn and Pontsticill. It then runs near the southern margin of the national park via Trefil and the Llangattock escarpment to Blorenge where it turns southwards. A very narrow ‘east crop’ and ‘south crop’ run by Cwmbran and north of Cardiff. It turns west again to meet Swansea Bay at Porthcawl. West of the bay, the rock forms the renowned southern coast of Gower between Mumbles Head and Worms Head. There are further occurrences in the Vale of Glamorgan, both inland and on the coast.

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.