The Geology of the Dolwyddelan Syncline (North Wales)

Abstract

The importance of the Dolwyddelan syncline from a stratigraphical standpoint has long been recognized. It was considered by Ramsay to be a vital link or ‘key-area’ in the supposed correlation of the Snowdonian country with that of Bala. More recently, it was learned that the syncline contained graptolite-shales intimately connected with the Snowdonian volcanic rocks and offering definite evidence of their age. Considering, further, the unfortunate circumstance that on Snowdon itself the highest lavas are also the topmost beds, and that the lowest lavas are separated by a thick group of sediments from the highest graptolite horizon yet recognized there, the need for a re-study of the Dolwyddelan syncline, especially for more precise evidence as to the age of the Snowdon lavas, becomes apparent. The limits of any syncline are, of course, arbitrary, and as our chief aim has been to examine the volcanic rocks and the sediments immediately associated with them, we have restricted ourselves to the re-mapping of a narrow zone extending for about a third of a mile on either side of the synclinal axis over a distance of some 6 miles. We have, in addition, made a reconnaissance traverse along the southern slopes of Moel Siabod in order to evaluate the relations of the Snowdonian lavas of the syncline to the Capel Curig lavas which underlie them. The area illustrated on the map (Pl. XXXV) is also depicted on the 1-inch Geological Survey map (Old Series), Quarter-sheet 75 N.E., and comprises the 6-inch Quarter-sheet Caernarvon-shire 23