October 2022 The Devonian Period 419 -359 Ma – Prof. Brian Georgious
a geological rock formation

Brian first explained the context of our local red sandstone as being one of six areas of the Devonian period within the British Isles. Its red colour is caused by haematite and reflects its formation in an arid environment within the Avalonia continent then situated about 30° south of the equator.

The paleogeography of the time included the first tree [Archaeopteris], and increasing atmospheric oxygen from plant proliferation, which in turn aided the development of terrestrial animals. Meanwhile extensive reef building in adjacent shallow seas saw trilobites, brachiopods and corals proliferating.

Meanwhile the Caledonian orogeny, a mountain building saga over a 100ma, created fold mountains of Himalayan size in Scotland and north west Britain. This was caused by the closure of the Iapetus Ocean through plate subduction, leading to the collisions of the continents of Laurentia, Baltica and Avalonia. This geologically fused Scotland and Northern Ireland with the remainder of the British Isles.

Brian then illustrated the resultant landforms we know today from the famous Siccar point in Northumberland to the folding at St Annes Head near Haverfordwest

Oct 2022 Geology 1(C)

​The Devonian Era also saw a famous mass extinction event which led to the loss of 75% of known species in the marine environment. The cause appears to relate to the deoxygenation of the sea bed.

Brian next turned to a second later Orogeny – the Variscan [419 to 299ma] – and its impact on the earlier Devonian material. Amazing folding occurred at places like Hartland Quay and Millhook Haven in north Cornwall.

Oct 2022 Geology 2(C)

To conclude Brian reminded us of the slippage problems in the 1960s on the new A40 just north of Monmouth and showed how it was overcome.

Altogether a Devonian grand tour!




Wed 12 Oct 2022


2:00pm - 3:30pm
Bridges Centre, Drybridge Park, Monmouth NP25 5AS
Jim Handley

Accessibility at Bridges Centre

Members’ monthly meetings are held at Bridges Community Centre, Drybridge Park. Some group meetings and activities also take place at Bridges. Off street parking is available here outside the building, and disabled parking is adjacent to the building entrance. There are no external steps or slopes, and the entrance doors are automatic. The ground floor is fully accessible and level throughout, and there is space for wheelchairs. There is a lift to the first floor, and accessible toilets on both floors. There is a hearing induction system in the Agincourt room where the monthly meetings are held.

Accessibility at Ty Price

Some group activities and meetings are held at Ty Price, St Thomas Community Hall, St Thomas’s Square. There is no off street parking here. The approach on foot is a gentle slope to double entrance doors. The ground floor of the building is fully accessible and there is a disabled toilet. The stairs to the first floor are wide and well-lit with a handrail on both sides, but there is no lift. There is a hearing induction system on the ground floor.

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