The Upper Ordovician Rocks of the South-Western Berwyn Hills

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Abstract
The area to be described is part of North-Western Montgomery- shire. All place-names mentioned will he found on the 1-inch Ordnance Survey Map, Sheet 136 (Bala). or on the accompanying sketch-maps (figs. 1, 2, & 3, pp. 488, 493, & 496). Geologically, this area is the western part of the southern flank of the great Berwyn dome of Ordovician strata; hut a glance at the geological map shows that in this south-western portion the beds are pinched up so as to form a wedge-shaped south-westerly extension, one flank of which runs with remarkable straightness for several miles. The reason for this straightness of outcrop lies in the fact that in this belt of country the strata are vertical, or have acquired a slight reversal of dip from the south-eastward overfolding. To this cause must also be attributed the parallel arrangement of hill and valley, each band of hard rock standing out as a straight ridge, and each soft bed forming a parallel depression. Thus the sandstones of the Caradocian and the grits and massive mudstones of the Salopian stand out as ridges, while the softer Ashgillian and the sliales of the uppermost Caradocian form the marked valley which is followed by the old pack-road connecting the Upper Vyrnwy valley with that of the Tanat. This strike-valley is drained by various streams. In its northeastern part to Pen-y-garnedd (Sheet 137 it is drained by a tributary of the Tanat, then for a short distance the headwaters of the Cain drain the

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