This section will discuss the geological history of the area for each lithological formation, as well as identify the main structures and what these were caused by.
The Millstone Grit at its base has an unconformity formed by uplift and folding of a northerly landmass. It has a thick series of quartzitic sandstones formed from weathering of an earlier landscape. There are several movements due to subsidence, which can be seen in the Millstone Grit through the basal grits. Following the Millstone Grit the coal measures were deposited with a gradual change due to increasing sea levels.
The lowest coal measures are classified at the base, by the shallowest water seen by the presence of plant roots. During the lower coal measures, large amounts of vegetation grew to enable the process of coal formation to begin. Plant growth became more prominent at the end of this period, shown by the significant coal seams at the top of the lower coal measures (Archer, 1968).
The middle coal measures were formed in a similar environment, they also contain marine bands such as the Upper Cwmgorse. These were formed as sea level rose and fell in the Carboniferous driven by Gondwana ice cap fluctuations producing marine transgression onto the South Wales Coalfield delta plain (Isbell et al, 2003). The surrounding sediments to the coal seams are generally mudstones with small amounts of sandstones. The depositional setting of the coal measures is an upper coastal deltaic flood plain with a dominant provenance direction from a southerly landmass (Archer, 1968). The middle coal measures have major fluvial channels flowing through them; these measures are constrained by cross fault activity and the presence of subsiding basement highs (Hartley 1993).
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...lor et al (1998). This plant matter is rapidly buried and compacted in anaerobic conditions forming a unique rock with several economic uses (Archer, 1968).
There are many depositional models of the formation of coal. The most relevant one to this area of study is the upper delta plain facies, as illustrated in Figure 2. This facies is characterised by coarsening upwards sequences of mudstone into siltstones then sandstones; these are known as para-sequences. Coal seams can be traced over large areas, which thin to poor quality coals. The formation of coal occurs as raised mires, which are locally eroded by channels known as washouts and are topped by sheet flood deposits (Guion et al, 1995). These channels are highly variable in size and are constantly changing making it hard to model without sufficient data, as the coal seams vary on a meter scale (Thomas, 2002).