Limestone and Coal Formation
The Carboniferous Period began with rising sea levels and the Abercrave ( Abercraf ) area sank beneath a shallow sea, which was colonised by corals, crinoids and other aquatic animals and plants. Later in the period, the seas receded and dry land reappeared, exposing layers of dead marine life which had accumulated on top of the sand. In time these layers became limestones layered on top of sandstones. Subsequent upheavals of the Earths crust tilted and thrust these rocks upwards to form hills and mountains, such as Cribarth, which gave rise to the name "Mountain Limestones". These provided a fertile base for the lush tropical forests which then engulfed the region.
The continuous cycle of life and death of the plants and trees in these forests built up thick deposits of rotting vegetation and large peat bogs formed. Inter-bedded with fluvial sediments, and buried progressively deeper, the peat was squeezed and compressed by the same enormous forces which caused the fault lines. The hard Anthracite, for which this part of Wales became renowned, was formed under this tremendous pressure in what we now call the "lower coal measures". Thick seams of coal were also formed in the later Westphalian sub-period but these did not become so deeply buried and were subjected to less compression, leaving them softer and with more retained volatile matter. We refer to these as the "upper coal measures" and they are typical of the main coal deposits across much of Europe.
Pictures from early 1900s
Clydach Vale Colliery >>>
Running from North to South and linking the A465 (Heads of the Valleys Road) to the M4 Motorway in South Wales are steep sided valleys with lofty grit stone edges. To the geologist these are the youth of the district. They contain the youngest rocks carved out by the most recent geological event, the last ice age, and are generally classified as the coal mining region of South Wales. The physiology of Mid-South Wales reveals two extensive valleys which follow a different line and these are the valleys of the rivers Neath and Tawe. The latter is known as the Swansea Valley.
The
Neath and Swansea Valleys, along with the "watershed" which they both share, are particularly
interesting to geologists because of the many different
rocks and formations exposed by fault lines.
The impressive water falls of Ystradfellte with their
sheer drops into deeply eroded basins are one of the more
obvious features of these faults. The two valleys are of
particular geological significance, as well as providing
spectacular scenic interest for all who visit them. They have been carved along two rather special fault
lines and it is not surprising therefore that they boast
many waterfalls, most of which can be accessed by established rights of
way.
The coalfields to the East lie in the centre of a Syncline, which is a bowl-like down-fold in the rocks, where it is common to find the youngest rocks in the centre, with progressively older rocks appearing as one moves further from the centre-line, or axis, of the fold. This is exactly what happens near Abercrave where the younger coal measures can be seen along the banks of the river Tawe and it's tributary the Llech near Henrhyd Falls at Coelbren.
Henrhyd Falls.
The progressively older Millstone Grit, Carboniferous Limestone and old Red Sandstone can all be studied between Abercrave and Glyntawe, where the ground waters feeding the upper tributaries of the river Tawe have carved out the magnificent caverns of The National Showcaves for Wales.
This erosion has taken place where the limestone closely contacts the old red sandstone and the calcium saturated water has produced many fine examples of stalagmites and stalactites.
The hills and mountains of the Brecon Beacons National Park to the North and Northeast have a red cast to them and the soils around Brecon itself are also a characteristic red in colour, which proclaims their ancient sandy origins.
National Showcaves for Wales
