Wales (Cymru) - Central and West Wales

This area contains the incised upland area of the Cambrian Mountains that extends southeast of the Bala Fault and northwest of the Welsh Borderland Fault, the latter is a network of major fractures that extend from Pontesbury in Shropshire southwestwards as far as St. Brides Bay in Pembrokeshire. The northeastern boundary of this area is coincident with the Welsh Border. No sedimentary bedrock lies on top of the basement rocks in this area.

Basement rocks

The youngest basement rocks form layers deposited between around 460 to 420 million years ago, comprising thick units of marine mudstone and sandstone. In parts of Pembrokeshire and the Berwyn Hills layers of volcanic rock are also present. Volcanic rocks are also exposed adjacent to the Welsh Borderland Fault at Llanwrtyd Wells, Builth Wells and Shelve. In the southwest of the area and in the Berwyn Hills to the northeast, mudstone, limestone, sandstone and tuffs were deposited between around 700 to 460 million years ago are exposed at surface and are thought to extend at depth beneath the area. The oldest rocks exposed in this area comprise intrusive igneous rocks, volcanic rocks and a range of sedimentary rock formed between around 700 to 600 million years ago and exposed in places within the Welsh Borderland Fault.

The basement rocks have been folded during earth movements and are often steeply inclined with a slate-like appearance. The rocks are typically strongly fractured and cut by many faults. Although there are no major aquifer rocks in the area, small scale groundwater abstraction for small private supplies from these heavily fractured rocks is widespread. The rocks of the Cambrian Mountains are host to Lead-Silver mineralization that was worked extensively until the early 20th Century. Gold mineralization at Dolaucothi was exploited sporadically between Iron Age and Edwardian times.