Pen-y-Glog Quarry

Highlights

Pen-y-Glog Quarry has yielded the oldest, well-preserved Prototaxites–Pachytheca assemblage in Great Britain, and the oldest preserved as petrifactions from anywhere in the world (Figure 3.6). It has also yielded a number of other enigmatic plant fossils, including Berwynia carruthersii Hicks. The assemblage provides a valuable insight into mid-Silurian floras, and into the nature of early land vegetation.

Introduction

This quarry [SJ 107 422], which lies on the north side of the River Dee valley, near Corwen, Clwyd, has yielded some of the oldest plant fossils in Great Britain. The fossils were described by Hicks (1881, 1882) and Dawson (1882). More recently, some poorly preserved material has been discussed by Burgess and Edwards (1988).

Description

There is no detailed stratigraphical section published for this locality. Approximately 30 metres of the Pen-y-Glog Slate Formation is overlain by 15 metres of the Pen-y-Glog Grit Formation (together, the Pen-y-Glog Group). The geological distribution of the two facies-associations is shown in (Figure 3.7). The Pen-y-Glog Slate consists of uniform dark grey shales, and has yielded a typical off-shore marine graptolite fauna of the Cyrtograptus murchisoni Zone (Elles, 1900). The Pen-y-Glog Grit consists of alternating coarse sandstones (with plant fossils), siltstones and shales, which have been interpreted as turbidites int^-Ong the Denbigh trough (Cummins, 1957), and which contain a Monograptus riccartonensis Zone fauna (Elles, 1900). The biostratigraphical evidence suggested is clearly indicative of a lower Sheinwoodian (early Wenlock) age.

The best preserved specimens are petrifactions from the sandstones, and include Prototaxites hicksii (Etheridge) Dawson and Pachytheca sp., together with some enigmatic spherical bodies. The underlying shales have yielded Berwynia carruthersii Hicks.

Interpretation

Other than some spores and cuticle fragments (Burgess, 1991; Burgess and Richardson, 1991), Pen-y-Glog has yielded the oldest known evidence of land plants. The best evidence is in the form of small fragments of Prototaxites hicksii, no more than 50 mm long. They show little of the gross morphology, but internal structure can be clearly seen in thin section (Hicks, 1881, pl. 25). It conforms with that normally associated with Prototaxites, consisting of wide and narrow sets of tubes, except the former are rather smaller and denser than in most other species (12–22 pm in diameter and c. 2500 tubes per cm² in cross-section). Barber (1892) suggested that it might be the same as Prototaxites storrei (Barber) Dawson, found in South Wales, but the smaller, denser tubes in the Pen-y-Glog specimens may indicate that they are different. According to Burgess and Edwards (1988), the thicker tubes may be internally thickened, in which case they would belong to their new form-genus Nematasketum. However, they were unable to confirm this in freshly collected material, and were unable to examine the original type specimens, and so made no formal proposal of transference. They are probably the oldest Prototaxites-typespecimens found in Great Britain to date. Arber (1904a) makes passing comments to other occurrences in North Wales but,. without further information about the localities, their age cannot be determined. The previously mentioned P. storrei specimens described from Rumney Quarry near Cardiff (Barber, 1892) are from the upper Wenlock (Cyrtograptus lundgreni Zone) and are thus younger. From outside of Great Britain, there is only one reliable record from older strata, from the lower Llandovery of Virginia, USA (Pratt et al., 1978), but this was based on tubes macerated from compression fossils. The records from the Ordovician of the Sahara (Arbey, 1973; Koeniguer, 1975) were based on compressions showing no internal structure to confirm the identification.

Associated with the Prototaxites are small spheres of Pachytheca. Hicks (1881) was able to describe some of their internal structure, but his illustrations are rather diagrammatic and the microscope slides are now lost. Barber (1889) reported examining the slides, however, and stated that the preservation was poor. He confirmed that they were Pachytheca, but could not place them in a particular species.

These are amongst the oldest specimens of Pachytheca found in Great Britain. The only slightly older specimen is from the lower Wenlock Buildwas Beds of Shropshire and is poorly preserved (Andrew, 1925). There are, however, several records of Pachytheca from slightly younger strata in the Wenlock Series of Wales and the Welsh Borders (Harris, 1884; Hooker, 1889; Barber, 1889, 1891; Thiselton-Dyer, 1891; Storrie, 1892; Seward, 1895; Strahan and Cantrill, 1912). Ritchie (1963) mentioned Pachytheca from possible Wenlock strata in Scotland, but did not illustrate the specimens. The exact age of these specimens is open to question.

A larger spherical body has come from the Pen-y-Glog sandstones (British Geological Survey collections, specimen no. zl 363); it is not well-preserved, but is larger (c. 8 mm in diameter) than the Pachytheca spheres found here, and does not show the characteristic two-layered structure of that genus. In a letter (dated 19th February 1946) kept with the specimen, W.H. Lang wrote that there was little doubt that it was a plant, but that it could not be identified beyond 'sphaerical body incerta sedis'.

The species from the sandstones are all of uncertain affinity, but both Prototaxites and Pachytheca are widely believed to be land plants. Their presence in mid-basinal marine sediments may be due to the sandstones being turbidites, the plant fragments having been transported into the deeper parts of the Welsh Basin by turbidity currents from a landmass, probably somewhere to the south.

Berwynia carruthersii Hicks (Figure 3.8) represents parallel-sided, sometimes branching axes, preserved as anthracitic coal (Hicks, 1882). Many have a rugose surface, which Hicks interpreted as spirally arranged leaf bases, but it is too irregular for this to be likely. Also visible are zones along the margins of the axes, which show a rather finer patterning. However, this and the irregular rugose patterning are probably just a taphonomic effect. In the absence of any internal structure being preserved, it is impossible to be certain as to what group of plants Berwynia belongs.

Hicks (1882) described some other enigmatic fragments from the Pen-y-Glog shales as Parka, but they are too poorly preserved for this to be confirmed.

Conclusion

Other than some spores, Pen-y-Glog has yielded the oldest evidence of land vegetation in Britain, about 427 million years old. The fossils are also the oldest-known land plant fossils in the world that show details of cell structure, but are not closely related to anything living today.

References