The Lower Marl, Sandstone-and-Marl, and Upper Marl groups comprising the Dittonian stage (Devonian) in Pembrokeshire, southwest Wales, are remarkably similar lithologically to the same strata of the Welsh borderland 100 miles to the east. The recurrent conglomerate-sandstone-siltstone cyclothemic series represent flood-plain deposits. The Pembrokeshire occurrence confirms the conclusion that the flood plain had a wider extent in Lower Old Red Sandstone time than previously thought.

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