SILURIA – The Story of a Lost Ocean
This was the third talk in the autumn 2011 series and was given by Prof Michael Rosenbaum of the Shropshire Geological Society. The subject of the talk was ‘Siluria: the story of a lost ocean’ and principally covered the area from Clee Hill in Shropshire to the Malverns in the South and to Welshpool in the west. The main part of the talk explained the history of geological discovery of this area (mainly Ludlow) from the 1760s up to the present day and the discovery of the ‘Lost Silurian Sea’
It was also explained why the area from Ludlow looking towards Clee Hill is of international importance as the rocks form an unbroken succession from PreCambrian (~600MYA) to the present. The rocks are mainly formed from marine deposits. This led to a meeting in 1979 of the International Union of Geological Sciences and the 1st meeting of the ‘Subcommission on Silurian Stratigraphy and the Ordovician – Silurian Boundary’. This Subcommission did not met in Ludlow for a further 35 years but has met again this year (2011).
In Ludlow the Castle and the Church have been built built from local stone and provide evidence of the local rock types. The castle is built on top of the sedimentary Whitcliffe Formation (WF) which is composed of Calcareous Siltstone. This type of rock is formed from mud deposited in a warm shallow sea. It has a pale grey colour when fresh, turning to brown when weathered and also becoming weak and crumbly. Whitcliffe Formation rock was quarried in the Ludlow area for 100s of years but the quarries have been out of use for over 200 years.
The Church was originally built in the 11th century from WF, still present on the north side. Sandstone was later used and was probably local but the actual source is not known. Additions in the 15th century used Felton Stone which comes from just a mile to the north. Like most Old Red Sandstone, it tends to crumble a little on exposure to frost and rain.
Repairs in the 1950s used stronger, but non-local, New Red Sandstone from Hollington quarry near Uttoxeter. These repairs now clearly stand out from the older rocks due to the contrasting strength, colour and texture.
The history of geological discovery in Shropshire and the eventual ‘discovery’ of the Silurian Sea goes back to the Georgian Period, around 1760. In those early days geologists were divided between the ‘Neptunists’ and the ‘Plutonists’. The Neptunists favoured a biblical explanation based on ‘Noah’s Flood’ while the Plutonists favoured a volcanic origin. This was also the time of the rise of the amateur geologist.
Amateur geologists at this time included John Whitehurst (1713-1788) who was a foremost scientist and engineer of his day. He was an advocate for the volcanic origin of the Derbyshire ‘Toad Stone’ and also drew up the first geological cross-section of Shropshire which would be recognised today. Also at this time Erasmus Darwin (1731-1802 and the grandfather of Charles Darwin) published his ‘Botanic Garden’ which included an early attempt to draw a geological cross-section of the whole Earth. Although incorrect in detail it did suggest that volcanoes were formed from the eruption of molten rock (lava) melted by the Earth’s internal heat. It also made a good attempt at explaining some of the Earth’s internal structure. Another amateur geologist of the time was Arthur Aikin (1773-1854). He was a ‘dissenting’ clergyman and later a chemist who rejected the biblical explanations of the formation of the Earth. He was a co-founder of the Geological Society of Great Britain in 1807. He proposed a survey of minerals for which he could get no financial backing, so it got no further although he did publish these ideas as a ‘Geological Map of Shropshire’ in the first Transactions of the Geological Society.
In the Victorian Period (1830–1890) the industrial revolution in Britain drove the need to identify coal and other mineral resources. This, in turn, required the creation of detailed geological maps. George Greenough and William Smith compiled the first comprehensive detailed maps and established the stratigraphy. This required detailed maps and these were prepared by the military (the Royal Engineers), later to become a civilian organisation (the Ordnance Survey). The first surveyors to carry out this work in Shropshire were the Wright brothers. Importantly, they noted local geological details and rock types; they even included cross-sections but these were not straight lines! This mapping in Shropshire was significant because the manuscript maps were lodged in the Geological Society in London and were seen by Roderick Murchison (1792–1871) before he embarked on his visit to the Marches to study the ‘Transition Era’ (TE), beneath the Carboniferous Coal Measures. Murchison had been encouraged by Sir Humphrey Davy to take up science, for which he had a natural talent, and it was his wife who introduced him to William Buckland at Oxford who was to pose the challenge of working out the geology of the ‘Transition Era’ (TE), greatly encouraged by his wife!
Murchison collaborated with Adam Sedgwick in this task with Murchison studying the TE from the top down and Sedgwick from the bottom up. Murchison came to Shropshire because of the Wright brothers’ maps of the area showing an apparently simple passage of Carboniferous strata down into the TE sequence. On his trip to the Marches he stayed with Thomas Frankland Lewis MP of Harpton Court in Radnor where he met Richard Banks, the botanist, who encouraged him to consult with local amateur naturalists interested in geology in and around Ludlow. Following this advice Murchison visited the Rev Thomas Lewis of Aymestrey whose fossil collection was from the Upper TE. Studying this collection, and the strata from which they were found, he saw the connection between the fossil evolution and the rock strata leading to his correct conclusion that the higher the rock layer the younger it had to be.
During the next year (1832) he met a second local amateur geologist, Dr. Thomas Lloyd, who was a keen fossil collector. His collection included primitive fish from the Old Red Sandstone which had until then been thought to have been laid down in arid desert-like conditions. This apparent anomaly was later to prove crucial to the unravelling of the TE.
The first occurrence of ‘fish’ remains was seen at the now famous ‘Ludford Corner’, the Ludlow Bone Bed, which Murchison used to define the Silurian/Devonian boundary. Here ‘Downton Castle Sandstone’ (deposited on land) overlies the older ‘Upper Whitcliffe Beds (deposited in the sea). This boundary horizon also includes plants, believed until recently to be the first evidence of life on land.
The end of the Victorian era saw renewed geological study, not least to resolve differences in opinion between Murchison and Sedgwick. In particular Tom McKenny Hughes, who succeeded Adam Sedgwick as the Woodwardian Professor of Geology at Cambridge, in collaboration with Charles Lapworth at Birmingham began extensive work at Comley Quarry where the oldest British Trilobite ‘Olenellus’ from the early Cambrian period (ca 540 MYA) was discovered. Using an all woman team of geology graduates from Cambridge the fossils, particularly the graptolites, in South Shropshire were studied and. Graptolites are better than trilobites for this purpose since they swim in open water and can therefore spread across deep seas, so can be used to correlate internationally.
Studies in South Shropshire in the 20th Century have further refined the stratigraphy revealing more of its complexity. In particular the relationship between Brachiopod fossils and strata has revealed much detail of the continental shelf that then existed on the edge of the Silurian Sea. The shallowest part was at the Malvern Hills getting deeper across Shropshire, deeper still (100 m) near Welshpool, and true ocean depths further west. These studies also show that stratigraphy is not just about rock sequences but also has to take into account evolution of fossil species as well as the effect of the environment. This leads to a complex picture of the play-off between the age of rocks determined by radiometric dating and the age determined by fossil evolution.
Recent work by the BGS over the past decade has been attempting to create a detailed 3D model of the area but still needs more data and funding to complete the field mapping. New discoveries are therefore still to be made and the Marches remain an internationally important geological area.
Some additional material can be found at:
http://geologymatters.org.uk/2011/07/21/siluria-revisited-2011
http://www.shropshiregeology.org.uk/sgspublications/Why%20Shropshire%20is%20important.htm