Wilton Bluff, Ross on Wye – the sixth in a series about top Earth Heritage sites in Herefordshire and Worcestershire.
Ian Fairchild writes: I used to take Birmingham University students on a sedimentological field weekend. We went each October in two groups a fortnight apart. Our destination was the Gower, to study the Carboniferous limestones and the modern sediments on the Loughor Estuary. But the essential stop on the way was the spectacular river cliff of Wilton Bluff overlooking the River Wye at Ross. And the mandatory task was to sketch the cliff and compare with the work of the master – in the expectation that some of his genius would rub off!
I am referring to John R.L. Allen, Fellow of the Royal Society, and a giant in the field of sedimentology, who died in October 2020, aged 87.
But first a biography of the cliff. Up to 15 m high, it runs for an extraordinary 1500 metres south-west from the town centre (includes 300 metres at the roadside) and represents erosion during a phase of incision (downcutting) of the River Wye. The rock unit is the Brownstones Formation, a 1 kilometre-thick series of river sandstones which formed in early Devonian times around 400-420 million years ago. Clearly Wilton Bluff is one of the biggest exposed rock faces in the whole of Herefordshire, but it also has an intricate internal structure at every point. For example, this photograph (of a view about 1.5 metres high) shows a complex series of dipping (cross-bedded) layers of sandstone, many with trains of pebbles, and the whole overlaid with intriguing colour-mottling. A treasure-trove of information lies here and the ideal person to decode it was John Allen.
Allen’s motto was that of Michael Faraday: “work, finish, publish”. His approach in the field was extremely systematic, filling his notebook with neat observations and sketches. He wrote many definitive papers and produced authoritative textbooks on sedimentary structures and the physics of sedimentation in which he complemented his flair for qualitative description with a mastery of quantitative analysis. Publish prolifically he did (there were rumours that his three initials stood for three different people) – initially as a geologist, later in archaeology, and up to 2019 in the field of landscape history. He was a dedicated educator at Reading University and I and my students benefitted from experiments that he recommended in one of his texts. The sight of wave ripples arising instantaneously in a tank rocked side to side is never to be forgotten!
Allen’s approach to the Brownstones can be glimpsed in the following illustrations. Take this section of Wilton Bluff photographed in 2019 with its complex stratification…and note the road sign.
Here is Allen’s sketch (including the road sign; by the way he hand-draughted all the diagrams in his papers himself). The essence of the changing Devonian river bed is laid bare. The river is covered by large flat-topped bars and on the downstream side of the bars, grains avalanche down a slope to form a cross-bed. The thicker lines define numbered sets of cross-bedding and each set corresponds to one bar.
Allen visualized the complex 3-D geometry of the river bed and its evolution in the sketch below – all part of his 57-page paper from 1983*, one of several single-authored papers that year. He also estimated a discharge of the Brownstones river(s) of 1250 to 3000 cubic metres per second, similar to the South Saskatchewan River in Canada – that would put it in the top 100 rivers today – a globally important flow. The paper also addressed a range of issues about the structure of river deposits which proved invaluable for the exploration industry. In summary: once John Allen tackled a topic, our knowledge of it was completely transformed.
*Allen, J.R.L. 1983. Studies in Fluviatile Sedimentation: Bars, bar-complexes and sandstone sheets (low-sinuosity braided streams) in the Brownstones (D. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-203.
A brief biography of John Allen is at https://www.reading.ac.uk/web/files/Graduation/Dec10_-_John_Allen.pdf .
We hope you are encouraged to visit Wilton Bluff by this article. Details of how to get there with some links for more background information are at: https://www.earthheritagetrust.org/sites-to-visit/ .