Pen-cerrig Stream Section

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Powys County

Habitats

Fast flowing river and stream

Geology & landscape

Rock Type: Limestone, Sandstone

Description

Description The basal Wenlock beds here consist of less than a metre of nodular, calcareous and ferruginous sandstone with traces of shelly fossils, perhaps the equivalent of the Acidaspis Limestone, overlain by black shales with C. murchisoni graptolites of the eponymous zone. The lowest beds of this biozone are rarely seen in the important Builth district. The locality also admirably demonstrates the occasional ‘mixed’ nature of the Builth Wenlock sequence, as odontopleurid trilobites and small brachiopods occur next to murchisoni graptolites on the same bedding plane, evidence of Builth’s position here at the ‘junction’ between basin and shelf. A key biostratigraphic locality for the Builth Wenlock. In layman’s terms, the interest of this site may be expressed more simply, and such a statement is provided below. This should not be taken as definitive and further information as to details of the interest can be obtained from the Countryside Council for Wales. The stream section at Pen-cerrig exposes a sequence of sandstones and shales which were laid down around 410 million years ago when much of Wales was covered by sea. At this time, north and mid Wales lay under deep water, known as the Welsh Basin, whereas the Welsh Borders and the Midlands of England lay beneath a much shallower sea. Such reconstructions of palaeogeography are based upon the observed distribution of rock types and fossils which are known to be diagnostic of these different environmental conditions. Detailed study of the strata exposed at Pen-cerrig suggests that this area lay near the junction of the deep water basin and the shallow water shelf during the Wenlock portion of geological time, about 410 million years ago. The site has yielded a rich assemblage of fossil animals and provides a valuable record of the marine fauna which lived at this time.

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