The basal Coal Measures of the South Wales Coalfield from Llandybie to Brynmawr.

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Abstract
Summary: The basal Coal Measures, from the Oastrioceras subcrenatum marine band to the Garw (Cnapiog or Bryn) Coal, have been studied in detail along that part of the North Crop of the South Wales Coalfield which lies between Llandybie in the west and the Clydach valley (near Brynmawr) in the east. This sequence falls mainly within the zone of Anthraconaia lenisulcata and its thickness ranges from 380 feet in the west to 130 feet in the east. Along much of the belt a lower arenaceous portion (the so-called " Farewell Rock ") is succeeded by a predominantly argillaceous division which contains a number of marine horizons. These bands can be identified by means of certain distinctive features. Many of them can be traced into the eastern areas and they show that the sequence is there attenuated and more uniformly arenaceous. This eastward thinning occurs even within separate units of the sequence. It can now be shown that the " Farewell Rock " of the western areas is older than sandstones which have been given this name in the north-eastern part of the coalfield. The basal Coal Measures of the North Crop are correlated in detail with those of other parts of South Wales and the variations in lithology and thickness are discussed. Isopachyte maps show that the present " structural " form of the coalfield was being moulded, through differential subsidence, in Lower Coal Measures times, with the maximum subsidence occurring in the Margam area. It is probable that these basal Coal Measures were never deposited over the Usk and Forest of Dean regions.