Moel Tryfan
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Geology & landscape
Mining / Industrial: Quarry · Geological Period: Cambrian, Quaternary · Rock Type: Conglomerate, Slate
Landscape processes: Glacial
Description
Description Moel Tryfan has been selected as an SSSI for its geological and geomorphological interests. This extensive site includes exposures of the lower Cambrian succession, stretching from the siltstones of the Tryfan Grit Group, through the Gilgwyn Conglomerate to the Striped Blue Slate of the Llanberis Slates. Exposures of the lower units are well shown on the summit of the hill, the higher units being superbly displayed in the quarry. The coarser greywacke horizons in the slates show a wealth of sedimentary structures (graded bedding, convolute lamination, cross lamination and various sole structures). The site is unrivalled in its exposure of the succession at these lower Cambrian horizons. The Cambrian Slate Belt is world famous for the high quality slate it has produced. Alexandra Quarry contains a range of lithologies and structures exhibiting the essential features of the Slate Belt. The overall structure exposed in the Quarry is a NE-SW trending, tight, upright anticline containing the Purple Slate Group in the core flanked by Dorothea grit and the overlying Striped Blue Slate Group. This broad structure is complicated by attenuation of the fold limbs, and the presence of strike faults. Detailed examination of strain markers and minor structures allows the intensity and mechanisms of deformation to be established. Moel Tryfan is a classic Pleistocene site of considerable historical importance for high-level shelly deposits described for the first time by Trimmer in 1831. In the 19th Century the site was central to the debate between the “diluvialists” and the “glacialists” and has remained controversial with respect to the origin and dating of the sediments. Conventionally a tripartite division of till, sands and gravels and till has been recognised. The sediments were first interpreted as the product of marine submergence during the Biblical Flood. Later, the effects of glaciations were recognised in the till units, whilst a period of marine submergence was invoked to account for the intervening shelly sands and gravels. Subsequently it was thought that the shelly deposits dated from an early advance of ice from the Irish Sea, surviving as high-level remnants of an old drift cover. More recently, radiocarbon and amino acid dating indicate that they are Late Devensian in age. Moel Tryfan played an important role in the development of the Glacial Theory and it continues to be a site of considerable interest for Pleistocene studies.