This large community spreads across both the Usk Valley and that of the upper Tawe.  Trecastle is the only place of any size though the hamlets of Llywel and Cwmwysg fall within its bounds.

The lie of the land

It ranges in height from about 200m beside the River Usk between Sennybridge and Trecastle and a similar height beside the Tawe at Craig-y-nos Country Park to 802m at Fan Brycheiniog  (SN 825217).  Other notable summits include:

• Fan Hir (760m) (SN 830208)
• Moel Feity (591m) (SN848230)
• Mynydd Bwlch-y-Groes ridge of Mynydd Epynt.  385m (SN 858342)

It is drained by NW flowing Nant Gwydderig and the SE flowing Afon Wysg / River Usk and its tributary the Afon Hydfer and also the Southward flowing Tawe and its tributary the Nant Tywynni.

Hard rock

The larger part of the area is underlain by the mudstones and sandstones of the Old Red Sandstone which are Devonian and upper Silurian in age. 

The southern fringes of the area are composed of Carboniferous Limestone.

Legacies of the Ice Ages

The east facing scarp of Fan Hir owes its existence to glacial action.  A dramatic and enigmatic moraine lies beneath the steep cliffs; its exact origins are still a matter of debate.

Llyn y Fan Fawr occupies a glacially excavated hollow in the sandstone rock beneath Fan Brycheiniog.

Much of the ground is covered by glacial till.

A number of large landslips have been identified in the upper Tawe catchment.

Archaeology

Archaeological sites include a pond bay and motte and bailey at Trecastle, a Roman road and a scatter of pillow mounds, standing stones, circles, house platforms and cairns on Y Mynydd Du.

Maps

Geology

Sheets 213 ‘Brecon’ & 231 ‘Merthyr Tydfil’ of the British Geological Survey’s 1:50,000 maps series of England and Wales give full coverage between them of both the bedrock geology and superficial deposits of the Community.  Sheet 231 is available in two editions.  BGS publish a 'memoir' and a 'sheet explanation' to accompany these maps.

General

The following Ordnance Survey maps cover the Community:

• Landranger sheet 160 'Brecon Beacons' (1:50,000 scale)
• Explorer sheet OL12 'Brecon Beacons National Park/Western area' (1:25,000 scale)

Public transport

Bus services operate along the A40 Llandovery to Brecon trunk road. The nearest railway station is at Llandovery on the Heart of Wales Railway line.

See Traveline Cymru for bus and train services including stops, timetables and route-planning information.