Geology
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 the Twrch Sandstone, also often still referred to by its earlier name, the Basal Grit. The less fequently 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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