Mynydd Llangynidr

Mynydd Llangynidr is a mountain in the Brecon Beacons National Park largely in the county of Powys, south Wales. Its southern slopes extend into the northernmost parts of the county borough of Blaenau Gwent. It is named from the village of Llangynidr which sits in the valley of the River Usk to the north of it. It is essentially an undulating plateau rising in the west to a height of 557m at Garn Fawr at OS grid ref SO 123151. A secondary high point is achieved at a point marked by a trig point at SO 147159. To the north the hill overlooks the valleys of Dyffryn Crawnon and Cwm Claisfer, themselves tributary valleys of the Usk. The shallow upper valley (Cwm Carneilw) of the Ebbw River reaches into the plateau in the southeast whilst the upper reaches of the Sirhowy Valley and its tributary the Nant Trefil define its western margins. A further tributary, the Nant Milgatw, reaches in from the south whilst the sharp edge of Rassau Industrial Estate also defines a part of its southern margin. Passing east across the B4560, the hill merges with Mynydd Llangatwg which has a similar character.

Mynydd Llangynidr is formed from a layer cake of Palaeozoic Era sandstones and limestones which dip gently southwards into the South Wales Coalfield basin. Broken cliffs of Carboniferous Limestone occur along the northern edges and this rock underlies the entire mountain. The plateau is formed from coarse sandstones ('gritstones') also dating from the Carboniferous Period and which have foundered in many places as the underlying limestone has dissolved over millennia. The larger part of the mountain has a pock-marked appearance due to the hundreds of shakeholes in its surface arising from the presence of the limestone beneath the sandstone cover. The frequently conglomeratic sandstones of the central and northern parts of the hill are the Twrch Sandstone, also often still referred to by its earlier name, the Basal Grit. The less frequently exposed sandstone of the southern part is the lowermost Westphalian age Farewell Rock which forms the base of the Coal Measures.

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