Mudstones above coals in the eastern part of the South Wales Coalfield have yielded diverse and well-preserved palynofloras. They indicate that the Llantwit No. 1 and No. 2 seams are Stephanian in age, and thus correlate with the Household Coals Member in the Forest of Dean. Until the formation of the highest coal seam in the succession (No. 1 Llantwit Seam), conditions were progressively becoming wetter, as indicated by an increase in abundance of lycophytes and a decline in the cordaites. However, after the formation of this stratigraphically highest coal, the lycophytes declined significantly, indicating that conditions suddenly became drier, perhaps as a result of uplift of the area as the Variscan Front to the south steadily pressed forward.

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