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The Grwyne Fawr is a river in the Brecon Beacons National Park in south Wales. A section of it forms the administrative border between Powys and Monmouthshire and also of the historic counties of Brecon and Monmouth. The river and its major tributary the Grwyne Fechan flow into the River Usk at Glangrwyney.〔Ordnance Survey Explorer map OL13 'Brecon Beacons National Park: eastern area'〕 The name probably originates from ''gweryn'' or ''gwerynau'' and ''mawr/fawr'' meaning 'large river at the wet place'.〔Owen, H.W. & Morgan, R. 2007 'Dictionary of the Place-names of Wales' Gomer Press, Ceredigion〕 The Gwryne Fawr rises at Blaen Grwyne Fawr on the southern slopes of Rhos Dirion in the Black Mountains and follows a southeastward course for several miles, its flow interrupted by the presence of Grwyne Fawr Reservoir, the only waterbody within this range of hills. Some way below the reservoir, Mynydd Du Forest clothes the sides of the valley. Though a public road penetrates the valley as far as the north end of the forest, the valley is very sparsely populated. It is only approaching the hamlet of Partrishow that the valley takes on a farmed appearance. The river now heads west along a valley whose alignment is guided by the presence of a geological line of weakness known as the Neath Disturbance. To the west, the Grwyne Fechan takes a parallel course to that of it larger neighbour. It rises on the southern slopes of Waun Fach and gathers a number of tributary streams from the broad ridge which separates it from the Grwyne Fawr. Though there are numerous conifer plantations, the valley has a more open aspect than that of the Grwyne Fawr. The two rivers join near the village of Llanbedr and head south as the Grwyne Fawr through the hamlet of Llangenny to their confluence with the River Usk at Glangrwyney. ==Geology== Both the Grwyne Fawr and the Grwyne Fechan drain the Old Red Sandstone hills of the Black Mountains. These are the Brownstones and Senni Formations which date from the Breconian stage of the Devonian period. The sandstones and mudstones of these formations can frequently be seen in the beds of the two rivers and sometimes exposed in the often steep valley sides. The lower reaches of the river run through an area blanketed with glacial till, a legacy of the last ice age.〔British Geological Survey 1:50,000 map sheets 214 'Talgarth' & 232 'Abergavenny' and accompanying memoir/sheet explanation〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Grwyne Fawr」の詳細全文を読む スポンサード リンク
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