Chwareli Hafod y Calch a Phlas Uchaf
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Geology & landscape
Geological Feature: Karst · Geological Period: Carboniferous, Silurian · Rock Type: Limestone, Sandstone, Shale
Landscape processes: Karst
Description
Description Chwareli Hafod y Calch a Phlas Uchaf lies about one mile west of Corwen in Denbighshire. The site lies within the Corwen Outlier, an isolated block of Carboniferous rocks surrounded by older Silurian strata. The disused quarries at Chwareli Hafod y Calch a Phlas Uchaf are of special interest for their Lower Carboniferous (Viséan) rock exposures. Geology: Rock exposures in disused lime quarries at Hafod y Calch and nearby Plas Uchaf are of national importance because they provide the only remaining accessible sections through the Lower Carboniferous (Dinantian Subsystem) Corwen Outlier. A richly fossiliferous sequence of limestone, sandstone and shale, possibly up to c. 250 m thick, has been attributed to the Loggerheads Limestone, Cefn Mawr Limestone and Minera formations which, based on macro-and micro-faunal evidence, span the Late Asbian to Brigantian stages of the Viséan Series. The site has a long history of stratigraphical and palaeontological research and is the type locality for the rugose coral genus Corwenia. It has also provided figured specimens of several other rugose coral species as well as biostratigraphically significant species of foraminifera hitherto unreported elsewhere in North Wales. Facies variations indicate cyclical marine deposition on a low-gradient carbonate platform influenced by glacio-eustatically and tectonically induced changes in sea level, with periodic subaerial emergence, karstic dissolution and pedogenesis. The Viséan succession was most likely deposited in a narrow gulf formed by localised and enhanced subsidence along the Bala Lineament and other linked structures and thus provides an important constraint on regional tectonic and palaeogeographical models. The recognition at the site of two of the main marker beds within the North Wales Viséan succession (the Main Shale and Coral Bed) together with a newly applied foraminiferal-and algal-based biozonation allow the outlier sequence to be correlated with other Viséan successions in the region and more widely throughout the British Isles.