sentences:  It is the sandstones and mudstones of the Lower Devonian Brownstones and Senni Formations, sometimes capped by the hard wearing sandstones of the Plateau Beds which form such striking peaks as Pen y Fan and Sugar Loaf and the dramatic scarps of the Black Mountains and Black Mountain.There are restricted occurrences of Devonian rocks on Anglesey too.The Carboniferous period is represented by extensive outcrops across South Wales from the Wye Valley in the east, through the South Wales Coalfield, the Vale of Glamorgan and Gower westwards to southern Pembrokeshire.