Mynydd Carn-y-cefn is the name given to the broad ridge of high ground between the Ebbw Vale

Ebbw Vale

Ebbw Vale is a town at the head of the valley formed by the Ebbw Fawr tributary of the Ebbw River, south Wales. It is the largest town and the administrative centre of Blaenau Gwent county borough...

 and the valley of the Ebbw Fach in the Valleys region

South Wales Valleys

The South Wales Valleys are a number of industrialised valleys in South Wales, stretching from eastern Carmarthenshire in the west to western Monmouthshire in the east and from the Heads of the Valleys in the north to the lower-lying, pastoral country of the Vale of Glamorgan and the coastal plain...

 of South Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. It lies within the unitary area of Blaenau Gwent

Blaenau Gwent

Blaenau Gwent is a county borough in South Wales, sharing its name with a parliamentary constituency. It borders the unitary authority areas of Monmouthshire and Torfaen to the east, Caerphilly to the west and Powys to the north. Its main towns are Abertillery, Brynmawr, Ebbw Vale and...

, formerly Monmouthshire

Monmouthshire

Monmouthshire is a county in south east Wales. The name derives from the historic county of Monmouthshire which covered a much larger area. The largest town is Abergavenny. There are many castles in Monmouthshire .-Historic county:...

.

The broad-topped ridge runs NNW-SSE and achieves a summit height of 550 metres (1,804.5 ft) at a point crowned by a trig point

Trig point

A triangulation station, also known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon or trig point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity...

 at OS grid ref SO 187084 between Ebbw Vale

Ebbw Vale

Ebbw Vale is a town at the head of the valley formed by the Ebbw Fawr tributary of the Ebbw River, south Wales. It is the largest town and the administrative centre of Blaenau Gwent county borough...

 and Blaina

Blaina

Blaina is a small town, situated deep within the South Wales Valleys between Brynmawr and Abertillery in the unitary authority of Blaenau Gwent, ancient parish of Aberystruth, preserved county of Gwent and historic county of Monmouthshire.-Notable people:...

.

Geology

The entire hill is composed of sandstones and mudstones dating from the Carboniferous Period. There are also numerous coal seams within the sequence, most of which have been worked. The upper part of the hill including the summit plateau is formed from the Pennant Sandstone

Pennant Measures

The Pennant Measures are a sequence of sedimentary rocks of the South Wales Coalfield. They are also referred to as the Upper Coal Measures and are assigned to the Westphalian 'C' and Westphalian 'D' stages of the Carboniferous Period....

, a rock assigned to the Carboniferous Upper Coal Measures

Coal Measures

The Coal Measures is a lithostratigraphical term for the coal-bearing part of the Upper Carboniferous System. It represents the remains of fluvio-deltaic sediment, and consists mainly of clastic rocks interstratified with the beds of coal...

.
The flanks of the hill owe their steepness in part to the action of glacial ice

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

 during the succession of ice ages

Quaternary glaciation

Quaternary glaciation, also known as the Pleistocene glaciation, the current ice age or simply the ice age, refers to the period of the last few million years in which permanent ice sheets were established in Antarctica and perhaps Greenland, and fluctuating ice sheets have occurred elsewhere...

.

There are numerous landslips on the flanks of the ridge, notably above Blaina

Blaina

Blaina is a small town, situated deep within the South Wales Valleys between Brynmawr and Abertillery in the unitary authority of Blaenau Gwent, ancient parish of Aberystruth, preserved county of Gwent and historic county of Monmouthshire.-Notable people:...

.

Access

A minor public road runs along the western edge of the plateau. Most of the ridge has been mapped as open country

Open Country

Open Country is a designation used for some UK access land.It was first defined under the 1949 National Parks Act , and was land over which an appropriate access agreement had been made...

 under the CRoW Act thus giving a right of access to walkers. There are a number of public footpaths and other public rights of way across and along the ridge.