Coed Glyn-môch Track

[SN 8158 3760]–[SN 8195 3765]

Introduction

This is an important reference locality in the northern part of the international type area for the Llandovery Series. It comprises a well-exposed trackside section on the western limb of the Cefn-y-gareg Syncline (Figure 3.23). The rocks exposed at this site belong to the Upper Llandovery (C) division of the Llandovery Series formulated by Jones (1925, 1949). Formal formation names were introduced by Cocks et al. (1984) to replace the letters and numbers used in the Llandovery area by Jones; for the upper Llandovery part of the succession the same formations were identified in the southern, central and northern Llandovery regions. These are the Rhydings, Wormwood and Cerig formations. As part of their remapping and revision, Cocks et al. (1984) logged a number of major sections, and the Coed Glyn-môch Track section is equivalent to their transect 13; it spans the entire thickness of the Rhydings and Wormwood formations and incorporates the lower beds of the Cerig Formation.

Description

The beds on this part of the western limb of the Cefn-y-gareg Syncline dip very steeply to the east and in places become vertical or overturned. The section in the track youngs to the east, and begins in the lowest beds of the Rhydings Formation. This formation comprises silty and sandy mudstones and is here more than 200 m thick (Cocks et al., 1984, fig. 4); the overlying Wormwood Formation is about 60 m thick and comprises well-bedded but poorly laminated muddy sandstones with considerable bioturbation. Jones (1949) showed a faulted relationship between beds now assigned to the Wormwood and Cerig formations, but Cocks et al. (1984) did not map a fault at this boundary. Only the lowest beds of the Cerig Formation are exposed; the lithology returns to silty mudstones, and Cocks et al. (1984) reported the occurrence of the graptolite ?Pseudoclimacograptus (Metaclimacograptus) sp. at this locality. Brachiopods can be found throughout the section, with the faunas comparable with those seen in the southern area (see site report for the Fron Road GCR site), but details of occurrences have not been tabulated in the literature.

Interpretation

The Coed Glyn-môch Track is important in that it provides a complementary section in the northern Llandovery area to the Fron Road site in the southern area. There are minor differences in lithological development and more striking differences in thickness. The Wormwood Formation is less strongly differentiated from the Rhydings and Cerig formations in the northern area (Cocks et al., 1984), and both the Rhydings and Wormwood formations are thinner. In transect d3 of Cocks et al. (1984), along the Fron Road, the Rhydings and Wormwood formations are 370 m and 108 m thick respectively; on the Coed Glyn-môch Track, the thicknesses are 220 m and 60 m. The Coed Glyn-môch Track section also provides a fauna of brachiopods and other shelly benthic fossils, showing local development of the Pentamerus, Stricklandia and Clorinda benthic communities. The sediments suggest a relatively deep shelf setting for the Rhydings Formation, a more oxygenated and perhaps shallower environment for the Wormwood Formation, and a further transgressive deepening for the lower Cerig Formation.

The exposures in the Coed Glyn-môch Track, combined with those at Scrâch Track, Trefawr Track and Cwm-Coed-Aeron, form a network that gives a complete coverage of the stratigraphical succession through the Llandovery Series in the northern part of the international type area.

Conclusions

This important locality in the northern part of the international reference area for the Llandovery Series, combined with other GCR sites, gives a complete coverage of the stratigraphical succession in this area. It exposes the Rhydings and Wormwood formations and the lower part of the Cerig Formation, and comparisons of sedimentology, stratigraphical thickness and faunal sequence can be made with the classic parallel section in the Fron Road, in the southern part of the Llandovery type area. As a key reference site in the international type area, the Coed Glyn-môch Track section is of major conservation importance.

References