The Silurian Rocks of the Talerddig District, Montgomeryshire

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Abstract
Summary: The paper deals with the ground between Dinas Mawddwy (Pugh 1928) and Tarannon (Wood 1906). The rocks of the area are grits, siltstones, shales and mudstones forming part of a greywacke suite. They are subdivided into the Gelli, Talerddig and Dolgau groups (Upper Valentian) and the Denbighshire Group (Lower Wenlockian). Some of the beds, for example the blue shales in the Dolgau sequence, are remarkably constant throughout the area, but in general, beds, members and groups exhibit pronounced variation in thickness and, to a lesser extent, in lithology. There is a general thinning eastward and southward, the latter being contrary to the regional attenuation towards the geosynclinal axis. The rocks are folded into a series of major and minor folds associated with which there is well-developed cleavage and jointing but little faulting. Three fold-sets are distinguished : a predominant NNE.-SSW. set, a less-pronounced E.-W. and an insignificant NW.-SE. set. The area is, therefore, dominated by the NNE.-SSW. structures prevalent in the Harlech, Plynlimon and Van domes to the west and south, but also contains some E.-W. structures related to the Berwyn dome to the north-east.

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