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
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.
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!
Jim
