The geology of the Prescelly Hills and adjoining areas in North Pembrokeshire and Carmarthenshire

Abstract

This thesis describes the geology of an area, over 85 square miles in extent, of Ordovician rocks. All the ground has been mapped on 6-inch scale maps, whilst mapping on the one-inch scale has been pursued westwards to correlate this with the Hayscastle Trefgarn Area (Thomas and Cox). The ground has been described under three sub-titles;- 1. "Prescelly Area". 2. "Imbricate Area". 3. "Western Carmarthenshire Area". Thrusts, hading to the N.N.W., traverse the country, and all the evidence available points to their having, at least, been initiated in pre-Bala times. The distribution of the rocks in the "Prescelly Area" is governed by the Nevern and Crug-yr-hwch anticlines, and the complementary Brynberian Syncline. Prominent developments of volcanic rocks occur near the base and at the top of the Lower Alanvirn succession. The "Imbricate Area" is complicated structurally and stratigraphically by the aggregation of thrust planes along this 'belt; of country. Within this narrow area two distinct facies of the Arenig, Llanvirn, and Llandeilo rocks are brought into juxta-position by south-eastwardly thrusting. "Western Carmarthenshire Area" is essentially the faulted and folded northern limb of the "St. Clears Anticline" (D.C. Evans, 1906). The rocks in this Area have an east-to-west strike, and a relatively simple structure. Still-like intrusions of 'spotted'-, and quartz- dolerites, keratophyres, a bostonite, and sheared granophyres, are prominent rock-types in the "Prescelly", and "Imbricate" areas. Intrusive rocks are unknown in the "Western Carmarthenshire Area".