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We now know that Sedgwick had taken on a task that could not really have been satisfactorily concluded given the state of knowledge of the time. If he had been in a position to base his Cambrian System on the strata of Baltic Russia, with its simple succession and fossiliferous rocks, the Cambro-Silurian 'problem' would not have arisen. Nevertheless, Sedgwick did start the long process of research into the geological structure and succession of the Cambrian rocks in North Wales. They are highly deformed, so much so that many of the fine-grained muddy seabed deposits have been compressed, cleaved and slightly metamorphosed into slates. Such processes destroy fossils and make the recovery of any that survive extremely difficult. Indeed, slates from the famous quarries of the Llanberis region are among the best in the world and were used to roof buildings all over Britain in the nineteenth century.

We also now know that the Cambrian strata of North Wales record a deepening succession of seabed deposits, from pebbly conglomerates and sands at the base followed by muds (now compressed into the Llanberis slates, some 1000 m thick), followed by a particular kind of sand layers called turbidites, which were laid down by sediment-laden submarine (turbidity or density) currents that flow over the seabed in a similar way to avalanches of snow. Above these relatively deep-water deposits are shallower-water sands and muds of the Lingula Flags.

Fossils are very uncommon in this region, especially in the lower part of the succession, and the first useful fossil remains are trilobites (such as Pseudatops

Sedgwick's tardiness in illustrating characteristic Cambrian fossils (e.g. these trilobites) was eventually overcome with the help of palaeontologists such as John Salter but it still took a long time before the Cambrian was restored to its rightful position as a valid system and period of Earth Time.

Sedgwick's tardiness in illustrating characteristic Cambrian fossils (e.g. these trilobites) was eventually overcome with the help of palaeontologists such as John Salter but it still took a long time before the Cambrian was restored to its rightful position as a valid system and period of Earth Time.

viola) preserved in some sandy layers within the Llanberis Slates. The sands are not so compressed as the muds and allow the preservation of these extinct marine arthropods, which were very diverse and evolved rapidly. Consequently, individual species are characteristic of relatively brief time intervals, which makes them very useful for matching contemporaneous strata between now separated outcrops.

The Lingula Flags preserve a greater variety of remains, ranging from the lingulid lamp-shells (brachiopods) after which the group of strata are named to several different kinds of burrows and trails made by worms and trilobites, all of which seemed to have lived in a shallow sea.

The whole succession is some 3000 m thick and must originally have been much thicker before the sediments were compressed. The fact that such a thickness of sediment, beginning and ending with shallow-water deposits, accumulated in this region shows that there must have been active subsidence of the seabed over a prolonged period. However, Adam Sedgwick would never have been able to believe the most extraordinary aspect of the Cambrian story of Wales. We now know that this Welsh Cambrian seabed originally lay many thousands of miles away from its present location (see p. 220), near the South Pole and attached to the northern margin of North Africa. But then it is only in the last few decades that the evidence for this extraordinary history has emerged.

Conquering the Devonian

Before relationships between Murchison and Sedgwick seriously deteriorated over the question of the identity and extent of the Cambrian, the two men added another prize to their collection of geological systems - the Devonian. For many years extensive ancient successions of sandstones, which are often distinctly red-brown in colour and make good building stone, had been called Red Rhab and Dunstone, or more commonly Old Red Sandstone by the early decades of the nineteenth century. There were extensive outcrops in Scotland, Wales and the Midlands, and there had been considerable confusion over their separation from a much younger series of similar strata known as the New Red Sandstone. Similar kinds of sediments occur in both series and both successions were relatively unfossiliferous, especially the latter. However, the river and lake deposits of the Old Red Sandstone were known to preserve the occasional remains of some very distinctive and strange-looking extinct fish-like animals, which were the subject of detailed study by the Swiss naturalist Louis Agassiz in the 1830s.

In the Welsh Borderland region the Old Red Sandstone could be seen to underlie and therefore predate the limestones and Coal Measures of the Carboniferous System. Murchison had used the base of the Old Red Sandstones

Right from the early decades of the 19th century strange-looking fossil fish and arthropods (e.g. Pterygotus) were found in the freshwater deposits of the Old Red Sandstone.

as a geological upper 'plimsol line' or datum from which he worked his way down into the older Transition rocks, which he was to rename as Silurian. So the Old Red Sandstone was fairly clearly defined as being older than the Carboniferous and younger than the Silurian. But there was a considerable problem and complication in the recognition of this sequence in southwest England.

Following their initially compatible and apparently successful campaign in the Transition rocks of Wales, Sedgwick and Murchison made a series of forays between 1836 and 1839 into the structurally complex geology of Devon, with its highly folded and faulted strata. Their interest in the region had been partly stimulated by some fossil plants from the region, which had been sent to them by Henry De la Beche. Like Murchison, De la Beche studied at the Military College at Great Marlow, where he received training in topographical surveying and mathematics, an expertise that helped secure his appointment as the first Director of the British Geological Survey in 183$, initially a one-man band. Prior to this he had, in 1832, started a personal investigation of the geology of Devon.

De la Beche inherited a sugar plantation in Jamaica, living there and studying the geology of the island for some years before returning to England in 1824 and settling down in Devon. That is when he started his geological investigations of the southwest, but with the abolition of slavery and a dwindling income from his plantation he no longer had independent means and so sought and obtained a government grant of £300 to produce geological maps of the region. De la Beche was an accomplished draughtsman, indeed he was also a skilful caricaturist who often made thumbnail sketches of his colleagues, especially during meetings of the Geological Society in London, which did not always enamour him to those who were the butt of his humour. They were sometimes worked up into lampoons and cartoons that were occasionally published.

De la Beche thought that the plants he had found came from 'old greywacke' strata and this aroused Murchison and Sedgwick's curiosity, because they had not found anything like them in the Transition strata that they had investigated in Wales. Murchison thought that if anything the plants most resembled those from the Coal Measures, and in 1836 he and Sedgwick confirmed his suspicions when they visited the locality from which they came in north Devon. But they were still intrigued by the highly contorted strata below the plant-bearing beds. They knew by this time that the Old Red Sandstone had not been identified in the region and this was a considerable puzzle.

Continuing their investigations in Devon, Sedgwick and Murchison could only think that the contorted rocks resembled those of Sedgwick's Cambrian 'territory' in North Wales. One difference was that in among the greywackes of Devon there are occasional fossiliferous limestones and, although many of the fossils have been distorted by the same earth movements that deformed the rocks, they were still possibly identifiable and many had been collected by local amateurs. To Murchison's eye they seemed to have a general resemblance to those of his Silurian strata, but were different in many details and he knew that he could not resolve the problem. However, Murchison was good at delegating such work and he knew a man who could help him - William Lonsdale.

William Lonsdale, 1794-1871, army officer (1812-15) who served in the Peninsular War and at Waterloo, then turned palaeontologist, became curator and librarian to the Geological Society of London (1829-42), studying fossil corals, especially those from Devonian and Carboniferous strata.

Lonsdale was another retired army man who had devoted many years to the collection and detailed study of fossils, especially the corals of the Mountain (Carboniferous) Limestone. He had also already helped Murchison with his Silurian corals and Murchison respected his judgement. However, Lonsdale reported in 1837 to Murchison that the corals from the limestones of Devon did indeed show some resemblance to those of the Silurian and also to those of the Carboniferous. To Lonsdale they seemed to be intermediate. Murchison's confidence in Lonsdale's judgement was shaken: he had certainly not considered the possibility that such contorted strata could be relatively young.

Murchison and Sedgwick continued to struggle with the structure and stratigraphy and collected more fossils, which were passed on to Lonsdale who found no new evidence to change his opinion. Eventually, Murchison and

Sedgwick had no option but to agree. After all, no matter how contorted the rocks were, their whole methodology rested on 'Smithian' principles of the identification of the relative age of strata by their characteristic fossils. There was a major implication to Lonsdale's palaeontological work: the Grauwackes of Devon with their limestones containing the marine fossils had to be of similar age to the Old Red Sandstone. It was one of the first occasions on which geologists realised that such drastically different kinds of strata were originally contemporaneous in their deposition.

Early in 1839, Sedgwick and Murchison together announced another new system of strata for which they proposed the name Devonian. Included were the slates, sandstones and limestones that had previously been referred to as Grauwackes within the Transition series of Devon and could now be seen to lie beneath the plant-bearing Carboniferous strata. Their characteristic fossils suggested that they must be younger than the Silurian and therefore equivalent to the Old Red Sandstone. But to fully justify the naming of a whole new system, the men realised that they needed to establish its applicability beyond the British Isles. Later that year they went to Germany, travelling through the Rhineland, Westphalia and the Eifel region, from where similar rocks and fossils had been reported. They needed to know if the fossils really were similar and if the strata that contained them were also bounded above by Carboniferous rocks and below by Silurian ones.

They collected plenty of fossils, which were shipped back to London to be examined by Lonsdale, George Sowerby, who had a wide-ranging experience of fossils of all ages as well as a specialist knowledge of molluscs, and Smith's nephew John Phillips, who was making a particular study of Devonian fossils. Their overall conclusions were that since the fossils were indeed similar to those of the British Devonian and the German strata were clearly overlain by Carboniferous-age rocks, they were also of Devonian age and that the System did indeed have a validity beyond Britain. The timely publication in 1841 of Phillips's book Figures and Descriptions of the Palaeozoic fossils of Cornwall, Devon, and West Somerset; Observed in the Course of the Ordnance Geological Survey of that District made an enormous contribution to the rapid acceptance of the Devonian as another subdivision of the old Transition Series. Phillips also took the opportunity to further argue the case for the usage of the terms Palaeozoic, Mesozoic and Kainozoic.

The name Palaeozoic, meaning ancient life, was first introduced by Sedgwick in 1838 as a grouping of two series of stratified rocks beneath the Old Red Sandstone. The younger of the two included both the Silurian and Cambrian, while the older group lay below and included highly contorted but still stratified strata. In 1840, Phillips had added the Devonian to Sedgwick's definition of the Palaeozoic and included his own new names for further groupings of younger strata - the Mesozoic Era (meaning middle life) and Kainozoic Era (meaning recent life). Then in 1841 he redefined the Palaeozoic to include the Carboniferous and Magnesian Limestone (which Murchison was soon to include in his new Permian System).

At the time the understanding of life during Palaeozoic times was that it included the origin of life itself, virtually all the sea-dwelling creatures from primitive forms such as sponges and corals to shellfish, including a lot of now extinct or uncommon groups such as the trilobites, graptolites and lampshells (brachiopods). Then there were primitive fish that arose in late Silurian and Devonian (Old Red Sandstone) times, while the first occupation of the land by plants and animals such as amphibians and reptiles was thought to have been a feature of the Carboniferous.

The Mesozoic Phillips defined as encompassing the Cretaceous, Oolites (Jurassic) and New Red Formation (Triassic). It was characterised by a significant change in life, with the extinction of some of the Palaeozoic groups of marine organisms such as the trilobites and graptolites, as well as the primitive fish groups and land plants. They were replaced by new kinds of fish both in the seas and freshwaters, new land plants, and importantly new large saurians (this was just before Owen called some of them dinosaurs), which occupied the land, took to the air (pterodactyls) and returned to the seas (the ichthyosaurs and plesiosaurs). Some of the earliest mammals had also been found, but little attention was paid to them.

The Cainozoic (as Phillips now spelled it) spanned the Tertiary and Recent, with the former including Lyell's Eocene, Miocene and Pliocene. The other Cainozoic divisions of the Palaeocene and Oligocene were yet to be established,

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