The Brecon Beacons of Old Red Sandstone are the highest (2907 ft.), but the Black Mountain bears a number of picturesque summits carved out of Millstone Grit and Carboniferous Limestone, which rise frequently over 2000 ft.
This coal-field occupies practically the whole of Glamorgan and part of Monmouth, and its surface slopes from the Black Mountain and Brecon Beacons to the sea as a gently inclined plateau, scored by deep valleys draining south.
The all-important Act of Union 1536 (27 Henry VIII.), converted the whole of the Marches of Wales into shire ground, and created five new counties: Denbigh, Montgomery, Radnor, Brecknock, or Brecon and Monmouth.
To S.E., and is itself adjacent to Aran-fawddy (2970 ft.), the highest point in the Cader Idris group. The system of Mid-Wales or Powys stretches from Cardigan Bay to the English border, and contains Plinlimmon (2462 ft.) in north Cardigan; Drygarn Fawr (2115 ft.) in north Brecon; and Radnor Forest (2163 ft.) in mid-Radnor.
The three combined ranges of the Black Mountains, the Brecknock Beacons and the Black Forest sweep across south Brecon from W.