Mynydd Merthyr is the name given to the broad ridge of high ground between Taff Vale

Taff Vale

Taff Vale may refer to:* The valley of the River Taff in south Wales*Taff Vale Railway, a Welsh railway line*Taff Vale Railway Co. v. Amalgamated Society of Railway Servants, a 1901 court case...

 (Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

 Cwm Taf) and the Cwm Cynon

Cynon Valley

The Cynon Valley , is a famous former coal mining valley within the South Wales Valleys of Wales. The Cynon Valley lies between the other mining Valley of Rhondda and the iron industrial Valley of the Merthyr Valley. Its main towns are Aberdare located North of the Valley and Mountain Ash located...

 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 forms the boundary between the unitary authorities of Rhondda Cynon Taff

Rhondda Cynon Taff

Rhondda Cynon Taf, or RCT, is a county borough in the South Wales Valleys of Wales. It consists of 3 valleys: the Rhondda Valley, Cynon Valley and Taff-Ely Valley...

 to the west and Merthyr Tydfil

Merthyr Tydfil

Merthyr Tydfil is a town in Wales, with a population of about 30,000. Although once the largest town in Wales, it is now ranked as the 15th largest urban area in Wales. It also gives its name to a county borough, which has a population of around 55,000. It is located in the historic county of...

 to the east.

The high point of 493m is at Mynydd Gethin (OS grid reference SO 044025) (though the summit 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...

 is at the lower height of 491m) is the culmination of a long ridge which extends northwestwards from the confluence of the Afon Cynon

River Cynon

The River Cynon is a river in South Wales and is one of the larger tributaries of the River Taff.It arises within Cwm Cadlan in the Brecon Beacons National Park as the Nant Cadlan and is supplemented by a number of small streams arising from springs in the Carboniferous Limestone and Twrch...

 and the River Taff

River Taff

The River Taff is a large river in Wales. It rises as two rivers in the Brecon Beacons — the Taf Fechan and the Taf Fawr — before joining to form the Taff north of Merthyr Tydfil...

 at Abercynon

Abercynon

Abercynon is a small village in the Cynon Valley in Mid Glamorgan, Wales. The unitary authority is now known as Rhondda Cynon Taff. It is composed of the village of Abercynon itself,Carnetown,Glancynon,Park View and Pontcynon. However, in recent years the sign to show motorists they are entering...

. The ridge includes the subsidiary summits of Twyn Brynbychan and Twyn Sych. To the northwest lies Mynydd Aberdâr.

Much of the hill has been planted with coniferous forest. Its flanks are heavily scarred by mining. The village of Aberfan

Aberfan

The Aberfan disaster was a catastrophic collapse of a colliery spoil tip that occurred in the Welsh village of Aberfan on Friday 21 October 1966, killing 116 children and 28 adults.-Mining debris:...

 sits at the foot of its eastern slopes, the scene of a tragedy on 21 October 1966 when a coal tip above the village slipped and engulfed the village school killing 144 people of whom 116 were children.

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...

.

Access

Virtually all of the afforested areas and some adjoining areas of moorland are classed 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 giving a right of access to walkers. In addition there is a limited network of public footpaths and other public rights of way which give access to parts of the hill.