On the Geology of Arenig Fawr and Moel Llyfnant

Abstract

I. I ntroduction . T he mountains known as Arenig Fawr and Moel Llyfnant are situated somewhat to the north of the centre of the county of Merioneth, and lie immediately east of the main water-parting of Wales. The area studied lies wholly within the quarter-sheets 13 S.E., 13 S.W., 21 N.E., & 21 N.W. of the 6-inch Ordnance-Survey maps of Merionethshire. Rising from a plateau some 800 to 1200 feet high, they attain heights of 2800 and 2437 feet respectively, and, being entirely above the cultivation-zone, are practically uninhabited. The Great Western Bala-Ffestiniog Railway crosses the northern end of the district, to which Arenig Station, close under the northern slopes of Arenig Fawr, affords easy access. Although to a certain extent glaciated, the mountains are swept clean rather than Drift-covered, and it is only in the lower ground that moraines become inconveniently abundant. The adaptation of surface-forms to rock-structures is exceedingly complete, and few are the places where the ribs of harder rocks do not protrude through the scanty soil or turf. Being open to the full blast of the westerly and south-westerly gales from the ocean, the district, receives its full share of weather; and, in consequence, although the eastern slopes are often grass- or heather-covered, the steeper parts of the western slopes are rugged and bare, and afford magnificent exposures of the component rocks. Further, the structure of the district is fairly simple, and the cleavage, though present, is not so strongly developed as to prevent one from breaking the