The Sratigraphy and Structure of the Cambrian Slate-Belt of Nantlle, (Carnarvonshire)

Abstract

The area described forms the south-western portion of the Cambrian slate-range of North-Western Carnarvonshire, and comprises about 10 square miles of the drift-littered country of Western Snowdonia. The hills on the north-west side of it consist of igneous rocks referable to some late pre-Cambrian volcanic group. The uplands on the south-east are of Ordovician sedimentary material, topped by the rhyolites which form the steep scarp-face of Garnedd Goch. Within this region, the general strike of the pre-Cambrian, Cambrian, and Ordovician rocks alike sweeps in a gentle curve from a north-easterly direction in the part of the slate-range north of the Llyfnwy, to a direction only a little south of west in the western portion of the district. This trend is nowhere in exact accord with the east-and-west course of the glaciated trough which contains the Llyfnwy valley and the lakes and village of Nantlle. As recorded upon the accompanying map (Pl. XVIII), exposures of solid rock are not particularly abundant in the area. Only locally (as, for example, on Clogwyn Melyn) are connected outcrops continuous over more than a few hundreds of square yards. Within the exploited slate-belt, artificial excavations furnish more than nine-tenths of the solid ground exposed, and even with these additions the total exposed area is only about a tenth of the infra-Drift extent of the slate-range. Fortunately, the depth and measurements mad during the survey of artificial exposures that the present conception of the structure of the region has been gained. The contoured index-map (fig. 1,