Treharris is a small town and community in the Taff Bargoed Valley in the south of the county borough (or principal area) of Merthyr Tydfil, Wales. It is located about 1 kilometre (0.6 mi) west of Trelewis, from which it is separated by the Taff Bargoed river, and 1.5 kilometres (0.9 mi) from Nelson in Caerphilly county borough. It had a population of 6,252 in 2001, increasing to 6,356 at the 2011 Census. As a community (or civil parish), Treharris includes the villages of Quakers Yard and Edwardsville. Due to steepness and narrowness of both the Taff and Taff Bargoed valleys at Treharris several notable bridges and viaducts have been built in the area.
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