Bryn-Llin-Fawr
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Geology & landscape
Geological Period: Cambrian, Ordovician
Description
Description This highly significant site contains a sequence through the sandstones and shales of the upper Ffestiniog Flags Formation, into the Cwmhesgen Formation (Dolgellau and Dol-cyn-afon Members) of upper Cambrian and lowermost Ordovician (Tremadoc) age. The transition from the Lingulella bearing arenities of the Ffestiniog Flags to the highly fossiliferous Parabolina spinulosa Zone of the Dolgellau Member is seen, beds near the transition having recently yielded an abundant and unique fauna of trilobites and bellerophontiform molluscs. The site contains a candidate international stratotype for the Cambrian/ordovician boundary and shows the most complete faunal sequence across the Merioneth-Tremadoc Series boundary known in Britain. The uppermost Cambrian Acerocare zone of the Scandinavian zonal scheme, hitherto unproved in Britain, is present and is highly fossiliferous, yielding a unique assemblage of trilobites, molluscs and brachiopods showing affinities with both Scandinavian and South American faunas. The lowest Tremadoc beds contain the earliest subspecies of the dendroid graptolite Dictyonema, facilitating correlation with the Scandinavian and New Brunswick sequences. The succeeding Dictyonema flabelliforme flabelliforme Subzone is well represented, yielding graptolites and an associated fauna of trilobites, hyolithids, sponges, brachiopods, molluscs and echinoderms. The site is of great research potential for its faunal and stratigraphic succession and the bearing these have on British and international correlation. Remarks: It is within the Snowdonia National Park.