Abstract

I. Introduction.

The district to be considered in this paper is situate on the southern flank of the Harlech Dome, and is underlain by strata of Upper Cambrian and Lower Ordovician age. The area is bounded on its northern side by the Mawddach Estuary, which separates it from the Harlech-Barmouth area of Lower Cambrian rocks. On its southern side the whole district is overlooked by the great escarpment of Cader Idris, from which, however, it is separated by a long straight fault-valley—the Llyn Gwernan Valley. The western boundary is formed by the valley of the Afon Arthog, while the eastern boundary lies near the town of Dolgelley. The area thus defined is about 8 miles long by 2 miles broad. The ground is represented on the Ordnance Survey maps, 1-inch (large-sheet series) 59 (Barmouth), and on the 6-inch quarter-sheets Merioneth 36 N.E., S.E., 37 N.W., with portions of sheets 33 S.W. and 37 N.E., S.W., and on the Geological Survey Map, Sheet 59 N.E. 1-inch (Old Series).

(a) Physical Features of the Area.

The ground between the two boundary-valleys, the Mawddach Estuary and the Llyn Gwernan Valley, is occupied by a hill-range, or rather by a series of ranges, made by the scarps of the various harder beds, and having a general west-south-westerly trend. The hills average 900 to 1000 feet in height, attaining their greatest, elevation on Bryn Brith (1259 feet). They fall into two main groups, which are separated one from the other by a broad upland hollow