An early Ordovician shore-line in Radnorshire, near Builth Wells

Abstract

Summary This paper deals mainly with the western margin of the Builth-Llandrindod Ordovician inlier and is concerned with the phenomena associated with the unconformity between the Newmead Series and the underlying Builth Volcanic Series. The rocks dip westwards and strike approximately north and south. They are extensively faulted and, in particular, the outcrops are displaced by powerful tear-faults. The Newmead Series consists of Grey Felspar Sands, Pyritous Felspar Sands and the flinty mudstones of Tan-lan. Within the sands, and especially in the lower part of them, there are many impersistent boulder-beds; both these rocks are beach deposits. There is progressive overlap within the Newmead Series, so that ultimately the flinty mudstone rest directly upon the Builth Volcanic Series. The Newmead Series is followed unconformably by the Glyptograptus teretiusculus shales. A rhyolitic series emerges from beneath the G. teretiusculus shales near Cwm-amliw and can be traced into the northern part of the inlier. The Newmead Series oversteps different divisions of the Builth Volcanic Series, the succession in the latter being most completely developed in the southern part of the inlier. The contact between the two series reveals the characteristics features of a coast-line, with wave-worn surfaces, sea-cliffs, screes and stacks as well as the beach deposits which occur in the Newmead Series. It is possible to reconstruct in detail the features of the area exposed to sub-aerial and marine erosion which was subsequently buried beneath the Newmead sediments. It consisted of horizontal layers of spilitic rocks, making trap topography, and in one locality this was overlooked by rounded hills of keratophyre. The presence of several outliers of Newmead sands to the east of the main outcrop indicates that the deposition of the Newmead sediments was probably widespread over most of the southern half of the inlier. From our observations, it is possible to interpret the general sequence of events, in particular of elevation, erosion and depression. It is shown that all these events took place within a relatively restricted part of the time-interval represented by the Didymograptus murchisoni zone. Finally, attention is called to the remarkable preservation of these ancient Ordovician physiographical features.