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April 12, 2018 at 8:10 pm #9490Anonymous
The Geology Group met at 10.30am on Wednesday 11 April 2018 at Merlin's Bridge Village Hall. This month's topic was the
DEVONIAN GEOLOGY OF BRITAIN.
When the Iapetus Ocean finally closed at the end of the Silurian Period continental collision between Laurentia, Avalonia and Baltica produced the Caledonian Mountains along a suture line that extended from Greenland and Scandinavia through Scotland and Wales, to Newfoundland and New England. Since these continental areas were situated near to the equator and cut off from rain bearing maritime influences, the climate became arid with low humidity and rivers fed by torrential storms often produced flash flooding. The rapidly eroding mountains provided a source of coarse sediments that were deposited in lakes and rivers in the inter- montane basins; these muds, sands and gravels eventually formed the rocks of the Old Red Sandstone. These eroded continental deposits were first laid down in Scotland in mid Silurian times but in the Brecon Beacons in South Wales deposition of the ORS did not begin until the end of the Silurian. However, whilst continental conditions continued throughout the Devonian Period (409-363 Ma) over most of Britain, a shallow shelf sea covered South Devon on the northern margin of the Rheic Ocean that separated the ORS landmass from Gondwana.
South West England. It is of interest to note that the Devonian Period was named by Sedgwick and Murchison in 1839 when they were working on the marine sediments in Devon and Cornwall. It was later realised that the continental ORS was of a similar age since in North Devon it is interbedded with Devonian marine sediments. The Foreland Grits (basal ORS) form steep cliffs near Lynmouth but these are succeeded by marine shales and then the Hangman Grits (Middle ORS) east of Combe Martin. Around Ilfracombe to the west, marine sediments outcrop but these are overlain by an Upper ORS outcrop that extends to the coast near Morte Point. In South Devon the Torquay Limestone is a shelf sea deposit formed of reefs containing corals, brachiopods and gastropods. In North Cornwall marine mudstones have been converted into slates by the Variscan orogeny and the famous ‘Delabole butterfly’ is an example of a brachiopod Spirifer verneuili that has been deformed by intense earth movements.
South Wales and the Welsh Borders. A large triangular outcrop of ORS extends from Shropshire south to the Severn estuary and west through the Brecon Beacons to Pembrokeshire. Up to 3000 metres of mudstones, sandstones and conglomerates accumulated on river flood plains and coastal lowlands bordering the Rheic Ocean. In the semi arid tropical climate thick beds of calcretes formed as evaporation brought carbonate minerals to the surface. Spore bearing plants grew near the rivers where primitive fish were living. The well known Ludlow Bone bed at the base of the ORS in Shropshire contains fish scales and spines in a condensed sequence only 4 cm thick. Cephalaspis is a typical armoured jawless fish found in the Lower ORS of the Welsh borders. In Pembrokeshire at Red Cliff, Marloes, there is a conformable junction between the Gray Sandstone Group ( Silurian) and the Lower ORS. Thick calcrete beds are present at Freshwater West and Chapel Point, Caldey Island.
Scotland. The ORS derived from the eroding Caledonian mountains, covers large areas in Scotland. In the Midland Rift Valley up to 9000 metres of sediment was deposited by braided streams rushing down from the surrounding highlands and forming alluvial fans that spread out at the foot of the valley sides (Highland Boundary Fault & Southern Uplands Fault). Extensive vulcanism also occured as evidenced by the basalt lava flows inter bedded with the Lower ORS sediments. The Ochils and Sidlaw hills are formed of these lavas
In NE Scotland the Orcadian Basin covered the area around the Moray Firth through Caithness to the Orkney Islands. This was the site of several large shallow lakes surrounded by alluvial plains and along the edge of the highlands alluvial fans developed where rivers deposited thick beds of sand and gravels. The Old Man of Hoy is a much photographed sea stack on Orkney where the ORS cliffs rise sheer from the sea. On the mainland the Caithness Flagstones are one of the most famous rock formations within the Middle ORS. They consist of banded and varved (seasonal deposits) lake sediments; rhythmic alternations of mudstones, dolomitic limestones and cross stratified sandstones. The carbonate horizons contain beautifully preserved freshwater fish remains, particularly in the Achanarras fish band. In 1839 Hugh Miller, a Cromarty quarry worker, discovered calcareous nodules within the ORS that contained armoured jawless fish like Pterichthys. He spent many years collecting fish specimens for local museums and in 1847 published an account of his work entitled ‘The Old Red Sandstone’. A small outlier of Middle ORS occurs at Rhynie in Aberdeenshire where volcanic waters rich in silica entered a peat bog and preserved a remarkable array of primitive vascular plants in chert.
Evolution at the cross roads? In Devonian times aquatic plants invaded the land and fish with lungs and bony fins developed into amphibians. The irony is that the Devonian has often been called the Age of Fish yet at that time 90% of Britain was occupied by the arid ORS continent.
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