Merthyr Tydfil County Borough has been one of the 22 unitary authorities in Wales since 1 April 1996 (though the area was originally granted county borough status in 1908). Merthyr Tydfil County Borough today has a population of 59,000. It is located in the historic county of Glamorgan and takes its name from Merthyr Tydfil town. The County Borough consists of the northern part of the Taff Valley and the smaller neighbouring Taff Bargoed Valley.
The first three maps are provided by A Vision of Britain through Time
These maps were found on Wikimedia Commons
These maps of Glamorgan post-1974 were found on another site and are very useful for sorting out the up-to-date geography of the area