Mountain Cottage Quarry
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Geology & landscape
Mining / Industrial: Quarry · Geological Period: Cambrian, Precambrian · Rock Type: Conglomerate, Granite
Landscape processes: Volcanic / Igneous
Description
Description Mountain Cottage Quarry is situated in North West Llyn, south of Tydweiliog, at the base of Mynydd Cefnamlwch. This site furnishes by far the best and most easily accessible exposures of basal Arenig sediments resting unconformably upon the Sarn Granite. It provides the clearest evidence for the pre-Arenig age of the Mona Complex in Llyn, particularly as the overlying Arenig contains not only fragments eroded from the underlying granite, but also clasts of mylonite derived from the Penmynydd shear zone which bounds the granite to the north-west. The quarry is also of historical interest, the unconformity having been misidentified as a baked, intrusive contact in the last century, this view persisting as late as 1936 when an unbaked basal conglomerate was first recognised lying above the granite. Subsequently, the granite was placed within the Mona Complex which was assumed to be Precambrian. Recent radiometric dating, however, has yielded a Cambrian age for the granite and opened the way for a reappraisal of the age of the Monian rocks on Llyn.