The Geology of Deudraeth. the Country between Traeth Mawr and Traeth Bâch, Merioneth

Abstract

Summary Strip outcrops of Upper Cambrian strata in normal succession form the southeastern half of the Deudraeth peninsula. Garth Grit of Lower Arenig age follows with slight discordance. In echelon against the north-western face of the grit ridge are slabs of strata varying in age from Lower Bala at Ty-obry to Lingula Flags at Rhyd. Beyond these towards the north-west is the broad outcrop of the Crushed Slates group, enclosing separated lenses of ash and lava of late Arenig or Llandeilo age. Slip surfaces in this crush belt cut across the regional cleavage and slice the aureole of the Tan-y-grisiau intrusion. Late dolerite sills have entered along shear surfaces, but themselves are uncrushed. The shear zone is the broken middle limb of the main Caernarvonshire syncline, here thrust south-eastwards to override the margins of the Harlech Dome. I. Introduction The country described forms the north-western fringes of the Harlech Dome, immediately to the east of the type area of Tremadoc (Fearnsides 1910A). It lies between the Traeth Mawr and the Traeth Bach. It is mostly included on the six-inch sheets Merioneth XI and XIX NW.¹ The only detailed stratigraphical account of the district is that of J. W. Salter (in Ramsay 1866). At that time thrust movements were not suspected, although the section he described includes country which is very severely disturbed. On his map of North Wales, D. Sharpe (1846) indicated an outcrop of greenstone extending from Moelwyn to the Portmadoc embankment close to the position of the grit