IN ANCIENT TIMES
Gradually the sea grew shallow. An upheaval also occurred to raise the neighbouring lands and to give rivers the opportunity of carrying mud, sand and pebbles into the shallower sea to form rocks called shales, sandstones and conglomerates. The considerable variations in these rocks, collectively known as Millstone Grit, are consistent with the conditions found at estuaries where the heavier materials are deposited first and the lighter particles carried further out to sea. The Millstone Grit occupies the area around Fochriw.
In some areas it is a thin surface over the Carboniferous Limestone but that rock is dissolved by water and in some places the underlying rock has been so removed that the Millstone Grit has collapsed in long hollows found, for example, in the area north of Rhymney Bridge. Some of this grit was quarried to be manufactured into fire bricks for lining blast furnaces.
In the period following, swamps and mudflats covered with luxuriant vegetation extended over the district. Forest debris slowly accumulated. As the area subsided into the sea, sands and muds were spread over the forest remains to seal them and preserve them as coal seams in which abundant fossils show us the types of vegetation that existed. Occasionally, some of the primitive ancestors of mussels, cockroaches and crabs were also sealed in.
Frequent changes of sea level caused a number of coal seams and layers of clay, shale and sandstone to be produced. The Pennant sandstone, a thick layer, was formed in this way and divided the Lower Coal Measures from the Upper Coal Measures.
The swamp waters also contained dissolved iron salts which were affected by the vegetation to form nodules, which resemble rusty pebbles, of iron carbonate. These nodules and the fine mud settling from the water gave rise to bands of clay-ironstone which were sufficiently rich to form iron ore.
The earliest industrial workings in the northern edge of the coalfield were based on the existence of this ore and of timber from local woods. The name of " Farewell Rock " has been given to the underlying surface of the Millstone Grit.
Later these Carboniferous rocks were compressed into a down-fold (syncline) by a mountain-building movement. A minor ripple in the basin structure has compressed the Upper Coal Measures in the region, one of the few regions of south Wales where they may be found.
South of the Millstone Grit the rock sequence is first, the Lower Coal, then the Pennant and lastly the Upper Coal. Both Coal Measures are easily eroded because they are softer and have so many variations in rock type. In this way, the Rhymney-Fochriw area and the Gelligaer-Blackwood district show much more open country. In between lies the harder, more resistant Pennant which practically makes walls of the valleys between Pontlottyn and Bargoed, Fochriw and Bargoed and Bedlinog and Trelewis. Iron ore, coal, fire-clay and, from the
The open country above Fochriw with the Tunnel Tavarn as was, which used to be Tunnel Cottages, in the distance
