A Ludlovian facies boundary in south Central Wales

Over much of its outcrop in south Central Wales, the earliest facies of the Ludlovian, comprising grey shaly mudstones, is overlain by an important turbidite facies. In the present paper these two facies are described and the boundary between them is fixed at the first appearance in the succession of numerous thick turbidite beds. While its horizon cannot be precisely defined with reference to the faunal sequence, there are grounds for believing that this boundary is not significantly diachronous in the area considered. Finally, some suggestions are made as to the possible significance of the facies boundary.