Cefn y Brithdir is the name given to the broad ridge of high ground between the Rhymney Valley

Rhymney Valley

The Rhymney Valley is a valley encompassing the villages of Abertysswg, Fochriw, Pontlottyn, Tirphil, New Tredegar, Aberbargoed, Rhymney, and Ystrad Mynach, and the towns of Bargoed and Caerphilly, in south-east Wales, formerly famous for its coal mining and iron industries.-Geography:Created as a...

 (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 Rhymni) and Cwm Darran 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 Caerphilly

Caerphilly

Caerphilly is a town in the county borough of Caerphilly, south Wales, located at the southern end of the Rhymney Valley, with a population of approximately 31,000. It is a commuter town of Cardiff and Newport, which are located some 7.5 miles and 12 miles away, respectively...

.

The northwest-southeast aligned ridge whose top surface is plateau-like, achieves a height of 477m at the summit of Pen Garnbugail (OS grid ref SO 100037). There are significant subsidiary tops of 446m to the southeast at OS grid ref SO 114049 and, to the north, of just over 460m at Mynydd Fochriw (OS grid ref SO 099047). To the south it drops away to the confluence of the Nant Bargod Rhymni with the Rhymney River

Rhymney River

The Rhymney River is a river in the Rhymney Valley, south-east Wales, flowing through Cardiff into the Severn estuary.The river forms the boundary between the historic counties of Glamorgan and Monmouthshire.-Path of the river:...

. To the northwest it falls to a broad saddle at around 360m between Pontlottyn

Pontlottyn

Pontlottyn |Caerphilly]], within the historic county boundaries of Glamorgan, Wales, United Kingdom. It is sited just to the south of Rhymney, and to the west of the Rhymney River.-History:...

 and Fochriw

Fochriw

Fochriw is a village located in Caerphilly County Borough, Wales. It was well known for its neighbouring collieries, which employed nearly the entire local population in the early 20th century. It lies within the historic county boundaries of Glamorgan...

.

Much of its western flanks are afforested with conifer plantations. There are numerous cairns, house platforms and other archaeological features scattered across the area. A Roman road

Roman road

The Roman roads were a vital part of the development of the Roman state, from about 500 BC through the expansion during the Roman Republic and the Roman Empire. Roman roads enabled the Romans to move armies and trade goods and to communicate. The Roman road system spanned more than 400,000 km...

 is recorded as crossing the eastern slopes of Pen Garnbugail.

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

A minor public road runs along the southern half of the ridge and this route is followed by the Rhymney Valley Ridgeway Walk

Rhymney Valley Ridgeway Walk

The Rhymney Valley Ridgeway Walk is a waymarked long distance footpath in the UK forming a circular walk in the Rhymney Valley area of South Wales.- The route :...

. Various other minor roads run up and across its northern reaches. Significant parts of the hill are moorland and have 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. Similar rights apply to many of the afforested areas. There are a number of public footpaths and other public rights of way over the hill.