In the hydroelectric project developed on the flanks of Plynlimon, about 10 miles east of Aberystwyth, Wales, a pressure tunnel leading off from the reservoir near Ponterwyd is driven through a succession of light- and dark-colored mudstones and black shales, all of lower Silurian (Llandoverian) age. Sedimentary bottom structures consist of sole marks of probable organic origin, flute casts, groove casts, and load casts. Current ripple marks and graded bedding are common. The folded structure and associated faulting are outlined.

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