The Upper Ordovician Rocks of the South-Western Berwyn Hills

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

The Ashgill Series near Welshpool, North Wales

“…In the southern Berwyns beds of Onnian age constitute the Nod Glas Formation, most of which comprises black graptolitic shale, but there they are succeeded by Ashgill sequences (King, 1923(King, , 1928Wedd et al, 1929;Whittington, 1938) which are normally thick and relatively complete. Near Church Stretton the youngest Ordovician sediments are of Onnian age and are exposed in the classic Onny Valley section where as the Onny Shales they cap a well-documented, marine, near-shore shelly sequence.…”

“…The nearest comparable Ashgill outcrops are those of the southern Berwyn Hills where, however, the sequence is much more complete than it is near Welshpool (King, 1923(King, , 1928Wedd et al 1929;Whittington, 1938). Preliminary work on the trilobite faunas of these successions by Price indicates that the base of beds currently taken as belonging to the Ashgill Series along the southern Berwyns outcrop strip is diachronous.…”

“…In the SW at Aber-marchnant (locality a I of King, 1923) the lowest Ashgill rocks are characterized by a fauna which includes Kloucekia robertsi (Reed), Tretaspis aff. radialis Lamont and T. hadelandica Stormer brachystichus Ingham.…”

“…Stratigraphic units used by previous authors (King, 1923(King, , 1928Whittington, 1938) have been based on attempts to recognize faunal divisions. Stratigraphic units used by previous authors (King, 1923(King, , 1928Whittington, 1938) have been based on attempts to recognize faunal divisions.…”

Notes on a temporary section in the ordovician at Conway, north Wales

“…Had phosphorus been present in any appreciable quantity, correlation might have been suggested with similar lithological sequences elsewhere in North Wales which also contain pyritiferous-phosphatic nodule bands. The most obvious resemblance would be between the Conway pyrite-nodule bands and the pyrite-phosphate nodule bands with a siliceous microfauna (Lewis 1940) at the base of the Pen-y-Garnedd (D. clingmi) black shales of Montgomeryshire described by King (1923 and1929). These are underlain by limestones, thin ashes and flags.…”