Harlech

Harlech and Iiongmynd Beds, the lower group of the Lower Cambrian system (q.v.), grey, purple, and red flags, sandstones and slates, with shaly beds and conglomerates, estimated at 4,000 feet thick in South Wales, on the north side of St. Bride's Bay in Pembrokeshire; at over 8,000 feet in North Wales, between Barmouth and Harlech, and in the slate district of Penrhyn and Llanberis; and at perhaps 25,000 feet in the Longmynd hills of Shropshire. The rocks of this series are in many places penetrated by igneous dykes, and exhibit distinct cleavage; but ripple-marks, sun-cracks, the impressions of rain-drops and worm-tracks (Arenicolites) indicate their shallow-water near-shore origin. They apparently rest unconformably upon Archeean rocks, the junction near Bangor being marked by a bed of conglomerate; but they pass conformably upward into the Menevian and Lingula Flags. Once thought unfossiliferous, they have yielded the oldest known assemblage of undoubted fossils, including a sponge (Protospongia), Lingulella ferruginea, Discind, and other brachiopo'ds, the pteropod Theca, and Paradoxides, Agnostus, and several other genera of trilobites, besides worm-tracks.