Abstract

D. K. Loydell writes: In a recent paper, McCann & Pickering (1989) describe a series of current ripples in the Wallog-Borth region in the northern part of the outcrop of the Telychian (Upper Llandovery) Aberystwyth Grits Formation. Whilst their current data provide evidence of derivation from the NNW (perhaps from the Irish Sea Platform), their stratigraphical conclusions are at variance with the BGS mapping of the Aberystwyth Sheet (see Cave & Hains 1986) and the author’s own palaeontological data.

McCann & Pickering state that the overall northward younging of the Lower Palaeozoic rocks in Central Wales suggest that the strata in the Wallog-Borth region are younger than those to the south. However, the overall structure of the Aberystwyth Grits is that of a periclinal syncline with those beds lying around Wallog and Borth dipping to the south. This is borne out by the fact that, in northern Dyfed, the underlying Borth Mudstones Formation crops out immediately to the north and east of the base of the Aberystwyth Grits. Palaeontological data provide confirmation of the southward younging here; collections made from south of Harp Rock (SN595872), which lies between Borth and Wallog, include Rastrites linnaei, Pristiograptus renaudi and Petalograptus conicus indicating a horizon within the lower half of the Monograptus turriculatus Zone, whilst at the northern end of Allt-wen (SN576796), 6 km south of Wallog, the presence of Petalograptus tenuis, Monograptus proteus and M. rickardsi minor is indicative of a level high in the turriculatus Zone.

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