Chwarel Ponterwyd (Ponterwyd Quarry)
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Geology & landscape
Mining / Industrial: Quarry · Geological Period: Silurian
Description
Description This disused quarry, situated close to the village of Ponterwyd, provides exposures of lower Silurian sandstones, siltstones and mudstones displaying very rare, small-scale structures representing some of the late effects of the intense crustal compression that took place during the Caledonian mountain building episode (orogeny). The rocks are transected by a first-generation cleavage (closely spaced parallel fractures), although no folds of this age can be seen here. Other possible early structures include fibrous quartz veins lying parallel to the rock layering. A number of examples of very rare second-generation folds can be seen disturbing the layering and first-generation cleavage. These are flat lying Z or S folds, sometimes with sharp 'chevron' forms; a weak second-generation cleavage is associated with the folds and, in some cases, sets of small quartz veins occupy the hinge zones. These exposures of second-generation structures provide a rare demonstration of the characteristics of the later stages of the Caledonian Orogeny in this region.