The Geology of the District between Llanymawddwy and Llanuwchllyn, Merioneth

Abstract

I. I ntroduction . The country described in this paper is situated in South-Eastern Merioneth, but a narrow strip extends over the county boundary into Montgomeryshire. The northern margin is an east-and-west line through Llanuwchllyn, and this is also approximately the southern boundary of the district recently described by Dr. G. L. Elles. The western limit is the high ground made up of Ordovician volcanic rocks: namely, Aran Mawddwy, Aran Benllyn, etc., and the eastern margin is roughly parallel, at a distance of from 4 to 5 miles. The southern boundary is about a mile south of Llanymawddwy, and is adjacent to the Dinas Mawddwy country. This district occupies an area of about 38 square miles, and is included in the 1-inch Ordnance Survey Map 136, in the Geological Survey Maps, Old Series, Sheets 60 N.W. & 74 S.W., and in the 6-inch maps, Merioneth, Sheets XXIX N.W., N.E., S.W., S.E., XXXIV N.E., S.E., and XXXV N.W., N.E., & S.W. It is a thinly populated region; Llanymawddwy on the south and Llanuwchllyn on the north are the only villages. Parts of the area are somewhat inaccessible, but the old mountain-road from Dinas Mawddwy to Bala over Bwlch y Groes (1790 feet above sea-level) passes through the district. There is comparatively little ground that can be cultivated, and the area is given over almost entirely to sheep-farming. The central part of the area is the watershed between three drainage-systems: namely, the Dyfi, the Dee, and the Vyrnwy (a tributary of the Severn). The