The Wye Gorge

View from Symmonds Yat Rock, Herefordshire.

The pre-Quaternary geology of the Symonds Yat area is outstanding as both the Devonian and Carboniferous systems are represented. The Lower Old Red Sandstone (ORS) is represented by the Brownstones Formation with the Quartz Conglomerate of the Upper ORS resting unconformably on this. The Tintern Sandstone is represented and is followed up the succession with the full sequence of the Carboniferous Limestone Series. The area provides excellent exposures of most of the formations with many of these now designated as regionally important. Of particular importance are the outcrops of Quartz Conglomerate, Lower Dolomite, Crease Limestone and Whitehead Limestone Formations on Little Doward Hill, Huntsham Hill, Coppet Hill and Kerne Bridge. The structure, illustrated by the associated bedding, provides excellent proof of the southwards dipping northern limb of the Forest of Dean basin.

The spectacular Quaternary features include entrenched meanders, solution caves, water worn high cliffs, limestone pavement and river terraces. The view from Yat rock is of one such meander, flowing between Huntsham Hill (Upper ORS) on the left of the photo and Coppet Hill (Quartz Conglomerate Formation up to Carboniferous Limestone series) on the right. In the distance the valley widens out onto the Brownstones Formation of the Lower Old Red Sandstone.

Two theories have been suggested for the development of the Wye Gorge. The accepted origin for the entrenched meanders of the Wye Gorge is that of superimposed drainage due to erosion during isostatic readjustment. The course of the Wye and its tributaries shows virtually no adjustment to the underlying geology or relief present today. This suggests that the river evolved on a higher strata of more recent rocks than are visible today, which have been removed by erosion, and the drainage pattern has been superimposed on the underlying strata (Miller, 1935). A recent alternative glacial theory has been put forward. Harris (2000) suggested that spillways from proglacial lakes could have been responsible for the dramatic downcutting of the river. He suggested that large volumes of debris-rich glacial meltwater could have cut the gorge we see today.

There is a high density of rock shelters and solution caves in the area, together with water worn high cliffs. King Arthur’s Cave (SO 545 158)(pictured right) is one such cave. It boasts Devensian fauna remains beneath a stalagmite floor (Barton, et al. 1997). Other caves in the vicinity include Little Doward Caves, Symonds Yat East Caves, Symonds Yat West Caves and Merlin’s Cave (Barton, et al. 1997).

Limestone pavement occurs on Little Doward Hill. Postglacial erosion of the near horizontal bedding planes has formed a smooth pavement. Solution activity has followed to form clints and grikes in the pavement. Deposits of tufa and travertine have also been recorded in the Wye Valley, near the Biblins Centre.

There is evidence of Quaternary deposition in the form of river terraces. Four terraces have been mapped by the BGS along the length of the Wye – at Bigsweir, Whitchurch and Dixton. The first terrace is discernible along much of the length of the Wye (Welch and Trotter, 1961). Terraces have been recorded at Kerne Bridge within the designated area.

The Silurian-Devonian boundary is actually situated within the Raglan Mudstone. The stratigraphy shown above is only a small part of that for the whole of Herefordshire. To see how the rocks found in the Wye Gorge relate to others in Herefordshire, click here.

References

• Barton, R.N.E., Price, C. and Proctor, C., (1997) The Wye Valley Caves project: recent investigations into King Arthur’s Cave and the Madawg Rockshelter. In Lewis, S.G. and Maddy, D., (1997) The Quaternary of the South Midlands and the Welsh Marches: Field Guide. Quaternary Research Association pp. 63-75.
• Harris, E., (2000) The formation of the Lower Wye Valley between Hereford and Chepstow and modification to the drainage patterns of the area during the Pleistocene era. Proceedings of the Cotteswold naturalists’ Field Club pp. 341-371.
• Miller, A.A. (1935) The entrenched meanders of the Herefordshire Wye. Geographical Journal LXXXV (2) pp. 158-178.
• Welch, F.B.A. and Trotter, F.M., (1961) Geology of the Country around Monmouth and Chepstow. Memoirs of the Geological Survey of Great Britain. HMSO

Howle Hill Quarry

West Quarry – The Causeway, Howle Hill, Ross-on-Wye from West side. June 1997. Field sketch by Rollo Gillespie.

The Carboniferous Lower Limestone Shales at Howle Hill are a series of transgressive marine and deltaic sediments of a back barrier and open shallow shelf environment. The overall trend of coarsening upwards is modified by several events. The Lower beds are mainly lagoonal laminated black carbonaceous shales containing the ostracods Kirkbyella and Beyrichiopsis glyptopleuroides, and also terrigenous plant material, mostly broken down into small cellular clumps.

View of the South face of The Causeway, Howle Hill Quarry.

Bed 12 is stromatolitic, of vertically stacked, linked hemispheroids. Several episodes of open shelf dominance, with a sparse brachiopod and crinoid population, are replaced by invasions of fine sediment and the return of the brackish back barrier conditions, (Beds 13-19) with Zoophycos and other trace fossils present also. Bed 23 is an apparent barrier washout, with a single trough cross-bed, ripple marks and thereafter a fully marine ecology with several types of brachiopods, crinoids, corals, bryozoa in abundance.

View of the East face of The Causeway, Howle Hill Quarry.

The Lower Dolomite, Whitehead Limestone and Drybrook Sandstone are absent at Howle Hill and the Lower Trenchard Coal Measures lie unconformably on the Lower Limestone Shales. This led to a mutually supportive mining/quarrying industry for lime burning in the 19th Century. Around 17 quarries were in use, and the Causeway is now the last remaining substantial exposure of the north edge of the Lower Limestone Shales which stretches through South Wales, Chepstow, the Forest of Dean and into South Herefordshire.