Twyn Llech, also known as Black Mountain, is a

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually highe ...

in the Black Mountains. It is the only Marilyn to fall exactly on the Welsh– English border, straddling Brecknockshire (currently administered as part of the unitary authority of Powys) and

Herefordshire Herefordshire () is a county in the West Midlands of England, governed by Herefordshire Council. It is bordered by Shropshire to the north, Worcestershire to the east, Gloucestershire to the south-east, and the Welsh counties of Monmouthsh ...

. Its

parent peak In topography, prominence (also referred to as autonomous height, relative height, and shoulder drop in US English, and drop or relative height in British English) measures the height of a mountain or hill's summit relative to the lowest contou ...

, Waun Fach, lies to the west.

Access

The Black Mountain is the highest point on

Hatterrall Ridge The Hatterrall Ridge (sometimes spelled Hatterall) is a ridge in the Black Mountains forming the border between Powys and Monmouthshire in Wales and Herefordshire in England. The ridge is about long, and is followed by the Offa's Dyke Path. On ...

Offa's Dyke Path Offa's Dyke Path ( cy, Llwybr Clawdd Offa) is a long-distance footpath loosely following the Wales–England border. Officially opened on 10 July 1971, by Lord Hunt, it is one of Britain's National Trails and draws walkers from throughout th ...

passes along the ridge, more or less from south to north. A steeper path leads to the summit from near the former youth hostel in the

Vale of Ewyas The Vale of Ewyas ( cy, Dyffryn Ewias) is the steep-sided and secluded valley of the River Honddu, in the Black Mountains of Wales and within the Brecon Beacons National Park. As well as its outstanding beauty, it is known for the ruins of Ll ...

to the west. The summit is unmarked and, because of the very shallow gradients along the summit ridge, virtually impossible to determine ''in situ''. Open access to all the moorland here means that deviation from the paths is allowed. The ground is peaty and normally very wet even in good weather, especially on the highest ground. It is the highest summit in England south of

Great Whernside Great Whernside is a fell in the Yorkshire Dales, England, not to be confused with Whernside, some to the west. Its summit is the highest point of the eastern flank of Wharfedale above Kettlewell. Great Whernside forms the watershed between ...

in the

Yorkshire Dales The Yorkshire Dales is an upland area of the Pennines in the historic county of Yorkshire, England, most of it in the Yorkshire Dales National Park created in 1954. The Dales comprise river valleys and the hills rising from the Vale of York w ...

, even though higher neighbours are very close by in Wales. However, some mountain lists, such as the

Nuttalls Nuttall may refer to: People * Nuttall (name) * Nuttall baronets Nature * Nuttall's oak, a fast-growing large deciduous oak tree native to North America * Nuttall's woodpecker, a species of woodpecker found in oak woodlands of California * Nutt ...

, consider the mountain belongs only to Wales, due to the Black Mountains range being principally a Welsh massif.

Geology

The mountain is composed of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s and mudstones of the Senni Formation of the

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...

which is of Devonian age. The shape of the

to the south and west of the Hatterall ridge strongly suggests that it was occupied by a glacier during at least one ice age, though not perhaps during the last ice age. The plateau was probably ice-free.

References

{{Authority control Marilyns of England Marilyns of Wales Hewitts of England Hewitts of Wales Hills of Herefordshire Nuttalls Mountains and hills of Powys Black Mountains, Wales Highest points of English counties