The Volcanic Series of Trefgarn, Roch, and Ambleston, Pembrokeshire

Abstract

The area dealt with in this communication and represented by the appended map (Pl. XL) is situated some 5 miles north of Haverfordwest, 7 miles south of Fishguard, and from 8 to 14 miles east of St. David's. About 25 square miles in extent, it lies immediately north of a district recently resurveyed by the officers of H.M. Geological Survey, and south and east of a tract of country surveyed by one of us (in collaboration with Prof. O. T. Jones) in 1912. It is occupied mainly by Arenig and associated Upper Cambrian rocks, of which the Arenig Series is of especial interest, since it contains two distinct sets of volcanic rocks—one of which immediately underlies the lowest visible Arenig sediments. In addition, an important series of rhyolitic lavas and ashes of uncertain age traverses the greater part of the district from Roch to Trefgarn. As is usual in Pembrokeshire, the general strike is in an east-north-easterly to west-south-westerly direction; but strike-faulting on an intensive scale has greatly disturbed the geological succession, and complicated the structure of the district. On the south, this area of Arenig and Upper Cambrian rocks is followed by extensive outcrops of the higher Ordovician beds of the Haverfordwest district, while its northern boundary is constituted, in large measure, by the pre-Cambrian massif of Hayscastle. Topographically, the district is rather more varied than is usually the case in Western Pembrokeshire. Considerable portions of the area are occupied by the 400-foot plain; but out of this Topographically