Welsh Borderland - Central Welsh Borders

This area includes most of the Welsh Borders region west of the Malvern Hills and south of the Shrewsbury to The Long Mynd area. As such, this area includes the major towns of Bishop’s Castle, Craven Arms, Clun, Ludlow, Knighton, Leominster, Hay-On-Wye and Hereford.

Sedimentary bedrock

The area surrounding the Black Mountains and around Pencoyd have a red-brown pebbly sandstone at the surface, commonly known as the Old Red Sandstone. Deposited approximately 400 million years ago, this unit is only seen in the Welsh Borders region at these two locations. This sandstone can reach a thickness of 200 m.

Basement rocks

Basement limestone and muddy limestone outcrop north-east of Ledbury. P212676.

The basement rocks in this area are folded and faulted and are varied and all over 415 million years old. Extending beneath the Old Red Sandstone and occurring at surface across much of the southern parts of this central area the bedrock comprises red-brown mudstone called the Raglan Mudstone. These mudstones are up to 800 m thick but are generally absent around Knighton, Clun and to the north-east of Presteigne.

In such areas the underlying, slightly older rocks, occur at the surface. They comprise dark-grey Silurian mudstones with rare sandstone layers. These dark-grey mudstones near Knighton are up to 300 m thick but they thicken beneath the red-brown mudstones seen in the southern part of the region and can descend to depths of up to 1000 m below the surface.

Wherever the dark-grey mudstone is observed, either at the surface or underlying other rocks in the southern part of the region, it is underlain by series of limestones and muddy limestones (Plate P212676). These limestones include the Much Wenlock Limestone that extends to around 550 m below the surface in the area around Knighton, Presteigne and Wigmore, and over 1200 m in the southern half of the region. Beneath the limestones exists a series of green to green-grey mudstones which date back to about 440 million years ago. In the area surrounding Knighton, Presteigne and Wigmore this mudstone is thought to extend up to 750 m below the surface. Underlying this is a series of rocks that date back to almost 500 million years ago. These rocks include a whole range of types including mudstones layered with dark volcanic tuff, sandstones and pale and dark-coloured lavas. These layers are thought to extend to a depth of over 1200 m in the area of Knighton, Presteigne and Wigmore.