Merthyr Tydfil is in Glamorgan.
Merthyr Tydfil is often referred to simply as Merthyr.
Previously, the town was a major centre of coal mining and steel manufacturing, and in the 19th century had a population that exceeded both Swansea and Cardiff. However, while those two cities have continued to thrive, Merthyr has been caught in a downward spiral of decline, and the town of today contends with the dual issues of mine closures and collapse of its manufacturing base. As the center of what has been called the UK's rust belt, Merthyr is sometimes referred to as the British Detroit.
It has one of the UK's best mountain biking parks at Gethin Woods. http://gethinmtb.com/
Merthyr Tydfil can be reached by A470 located 22 miles north of the M4 Junction 32 which is located just north of Cardiff. The A470 connects with the "Heads of the Valley road" which connects Swansea with the M5. The road is good quality dual carriageway between Merthyr and Cardiff.
Merthyr has it's own railway station and there is an half hourly service between Merthyr and Cardiff Central Station.
Merthyr Tydfil makes an excellent base camp for those wishing to see the Brecon Beacons without being cut off from transport and civilisation by staying in Brecon.