IN ANCIENT TIMES
Pennant, building stone, from which most of the older properties are built, were provided by these rocks.
Thus the geological topography of the "heads of the valleys" region was well suited to the production of iron since iron ore, coal and limestone were in ample supply.
As previously mentioned, the south Wales coal-field was laid down above thick limestone and millstone grit deposits which came to the surface at the south Wales coastline and to the north of Merthyr Tydfil. The strata took on the shape of a deep anticline and synclines underneath the valleys, which had its deepest point in the middle valley regions.
Where the limestone outcropped, above it were the iron ore and coal deposits and "opencast" mining became much in evidence. The initial method of winning these materials was by "scouring" in that the raw materials were dug out of the ground, conveyed to a water source and washed, thus separating the clay and earth prior to being transported to the furnace site for smelting. The areas in which these activities took place are called "patches" some of which are still in evidence today.
Shaping the Land.
One problem that emerges from the preceding facts is how to account for the southward flowing rivers across the coal basin where a central lake should be expected. It can be explained by the probability that most of present Britain was submerged under a sea in which chalk was deposited. This period, about 65 million years ago, is called the Cretaceous Age.
When the land rose again, the rivers flowed southwards down the slope of this fresh rock. Finally the chalk covering was eroded away and the underlying carboniferous rocks and its structures were revealed, but the river system of the newer, higher and younger rocks had been " superimposed " upon south Wales.
A long time later, in Pleistocene times, which began about one million years ago, Arctic conditions prevailed over most of north-western Europe. Snow piled up to be converted into ice. At first it collected on such high ground as the Brecknock Beacons and then it started to slide down the existing valleys.
Soil and loose stones were removed and transported to other localities. That is the reason for red boulders of Brecknockshire rocks being found on the edge of Cefn Brithdir. When the ice finally melted about twenty thousand years ago considerable amounts of glacial material were left behind.
At Bargoed, two glaciers met and left a shoulder of medial moraine which now secures the northern end of the railway viaduct. Boulder clay lined the valleys. Any stream, such as the one which flows from from Mynydd Fochriw to the valley bottom at what was called Cwm Bridge just south of Pen-y-Banc, at the entrance to the Parc Cwm Darran Visitors Centre, shows this mixture of stones and clay. The U-shaped valleys produced were considerably deeper, straighter with flat floors and steep sides.