In 1540, an English traveller, John Leland, spent some time travelling through Wales. Luckily, he kept an account of his journey, the relevant part about the Merthyr area is transcribed below.
“Merthyr Tydfil is in the commote of Senghenydd Uwch Caiach which is in the cantref of Eweinlwg. To go from east to west in the highest part of Glamorganshire towards the roots of the Black Mountains, is a sixteen mile of wild ground almost all.
Uwch Caiach stretcheth up to Taf by the east bank from Caiach to Morlays Castelle (sic), and two miles upward by north-north-east to Cae Drain, where the boundary is between Upper Monmouthshire, Breconshire and the Uwch Caiach part of Senghenydd.
Morlays Castelle standeth in a good valley of corn and grass and is on the right bank of the Morlays Brook*. This castle is a ruin and belongs to the King. Morlays Brook……comes out of the Breconshire hills, near Upper Monmouthshire and to Morlays Castelle, and about a mile lower in the parish called Merthyr it goeth into the east bank of the Taf.
There is a hill called Cefn Glas**, and stands between Cynon and Taf. This is the boundary between Miscin (sic) and Senghenydd. The ground between Cynon and Pennar is hilly and woody.
The water of the Taf cometh so down from woody hills and often bringeth down such log and trees, that the country would not be able to rebuild the bridges if they were stone, for they are so often broken.”
* Actually the Taf Fechan River
** Part of the Aberdare Mountain which overlooks Quakers Yard