Fan Brycheiniog is the highest peak at a height of (above sea level) in the Black Mountain (Y Mynydd Du) region of the Brecon Beacons National Park in southern Wales. There is a trig point at the peak and on the edge of the escarpment, and nearby, a stone shelter with an inner seat. It lies just inside the historic county of Brecknockshire (now part of the Powys council area) which gives the mountain its Welsh name. A subsidiary top (Fan Foel), less than a kilometre from the summit along the ridge to the northwest, is the highest point (county top) of the neighboring county of Carmarthenshire. Fan Brycheiniog is also within the Fforest Fawr Geopark designated in 2005 in recognition of the area's geological heritage. The views of the moorland and open country to the north are spectacular when the weather is clear, and reveals the isolation of the range, especially when compared with the more popular Pen y Fan range to the east. Access The Beacons Way, a waymarked long distance footpath heading southwest from Llanddeusant passes along the summit ridge before descending to the southern end of the lake Llyn y Fan Fawr to the east of the summit, en route to Abercraf in the southeast. The path crosses several peat bogs, but there are stone pavements in many places to protect the walker from the wet conditions. There is a stone staircase of regular downward gradient across the escarpment to the lake below. It is easily followed, and is dry underfoot when the weather is clear. The path leads on to Picws Du and Waun Lefrith above the smaller glacial lake of Llyn y Fan Fach. Fan Foel and these two peaks form the so-called Carmarthen Van, since they fall within Carmarthenshire, which has its border with Powys at Fan Foel. Geology Fan Brycheiniog is formed from the sandstones and mudstones of the Brownstones Formation of the Old Red Sandstone laid down during the Devonian period. Its summit and southern slopes are formed from the hard-wearing sandstones of the overlying Plateau Beds Formation which are of upper/late Devonian age. The cwm below the summit drains into the River Usk to the north. The southern slopes drain into the Afon Twrch and the slopes to the east drain into the River Tawe. The local soils are thin and acidic owing to the large areas of peat bog