A multi-layered dolerite complex of laccolithic form, Near Llandrindod Wells, Radnorshire

Abstract

The paper gives an account of an area near Llandrindod Wells, Radnorshire, which includes albite-dolerites. It was mapped by the Geological Survey about a hundred years ago as a single intrusion of roughly triangular form, two miles long and three-quartors of a mile wide, trending in the direction of the general strike of the country rocks. Detailed mapping on the scale of 25 inches to the mile has shown that the area includes no less than 315 dolerite masses, each surrounded by black shales of the Llandeilo zone of Nemagraptus gracilis . Several quarries clearly that there is no difficulty in mapping the separate intrusions. If an envelope were drawn so as to surround the area containing dolerites, its form would suggest comparison with a laccolith, but the envelope contains only about 25 per cent of dolerite, the rest being shale. The name laccolith is applied to the Llandrindod intrusion because of the occurence of many forms transitional between it and the ideal laccolith of Gilbert. One of the remarkable features of the area is that quarrying has disclosed several small plug-like masses at right angles to the bedding in the shales above which the dolerites have been removed. It is evident that these served as feeders to the over-lying dolerites. Small masses of oval or circular outline occur among larger ovoid masses and from their size and form these also are believed to be feeders; over 100 of them have been counted. The area has been tilted, so that the rocks now dip in a northerly or north-westerly direction at about 40°.