The Cwm Llwyd Outlier of Namurian Middle Shales in the central part of the Black Mountain escarpment of SouthWales is an\ud unusual structure. It occupies a topographic depression located more than 2 km from the main outcrop of the Middle Shales and\ud has a stratigraphic discordance with its wall-rocks of more than 300 m. Previous interpretations advocating a karstic origin imply\ud that this is the deepest known subsidence in Great Britain. However, although the outlier lies among some of the finest epikarst\ud landforms in the country, several factors indicate that the subsidence was not caused by a karstic mechanism. These include the\ud amount of stratigraphic displacement in contrast to the depth of available vertical space in the putative host Carboniferous\ud Limestone, the relatively simple form of the outlier, and the absence of brecciation, marginal solution residues and associated\ud palaeoendokarst features in the adjacent wall-rocks.\ud The outlier is re-interpreted here as the product of faulting which occurred during a Variscan strike-slip or transpressive\ud tectonic regime, producing local sinistral transtension that may have been re-activated in the Palaeogene period. Remapping the \ud outlier at a scale of 1:2 500 indicates that the present structure is related principally to a left-stepping offset between the Cwm\ud Llwyd and Llwyn Celyn faults. Analogies are drawn with the Cainozoic pull-apart structures in Southwest England. Near-surface\ud reddening and kaolinization of the beds probably represent pre-Pleistocene weathering
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