The Bala and Llandovery Rocks of Glyn Ceiriog North Wales.

Abstract

1. Introduction . It was in continuation of the work which we had begun at Corwen that in 1892 we paid our first visit to Glyn Ceiriog. At Corwen we had been unable to detect any decisive proof of unconformity at the base of the Corwen Grit; but in the Bala Beds below we did not find the characteristic fossils of the Sholeshook Limestone or of other Upper Bala deposits, and were therefore led to believe that the highest beds of the Bala Series were absent, and that there was probably an unconformity which was too slight to be visible in so small an area. The section at Glyn Ceiriog encouraged the hope that higher Bala Beds might there be found, and it seemed, moreover, to afford excellent opportunities for the examination of the whole Bala Series. To judge from the map of the Geological Survey, or from a cursory inspection of the ground, it would be difficult to imagine a locality more favourably situated for the purpose. The valley of the Ceiriog appears to cut through the whole of the Bala Series, from the lowest ash-band up to the Glyn Grit; and, owing to the steepness of its banks, the rocks are admirably exposed, while most of the beds are fossiliferous almost to excess. It was not long, however, before we found unexpected difficulties. The same succession, or rather the upper portion of it, should appear in the nearly parallel valleys of Tyn-y-twmpath and Nantyr; but when we examined those

The caradocian rocks of the north and west berwyn hills, north wales

“…This is a sequence of bedded acid tuffs (Groom and Lake 1908). The sequence is thickest in the east and thins westwards.…”

“…In the north a rather monotonous mudstone and siltstone succession can be divided into two formations, the Teirw Siltstone Formation and Bryn Siltstone Formation separated by the intervening Pany Tuff Formation (Groom and Lake 1908). In the west the Pandy Formation is absent so a similar division cannot be made.…”

“…The Pen-plaenau Formation represents the lower part of the Teirw Beds as described by Groom and Lake (1908) and includes the mudstones and interbedded thin laminated siltstones and sandstones between the Cwm Clwyd Tuff Formation and the Swch Gorge Tuff Formation. The formation thickens westwards from about 150 m near the Ceiriog Valley, to 215 m at the Swch Gorge and about 350 m in the west Berwyns.…”

“…Graptolites are recorded by Groom and Lake (1908) and a rich shelly fauna occurs in the upper levels of the tuff at locality 877. The fauna, including Broeggerolirhus broeggert FIexicalyrnene (Reacalymene) limba, Basilicus sp., Dinorthis c f. bemynensis and Reuschella oblonga indicates a lower Soudleyan age by comparison with the Shropshire succession.…”

“…As originally described by Lake and Groom (1908) the Teirw Beds included the beds between the Cwm Clwyd Tuff and the Pandy Tuff i.e. the Pen-plaenau Siltstone Formation, the Swch Gorge Formation and the Teirw Siltstone Formation.…”