Devonian To Cambrian

The distant hills of Wales can just be seen from the lowlying plains of the English Midlands. The Borderlands between the two countries have been greatly disputed over the centuries, although today they are occupied by picturesque market towns such as Oswestry, Shrewsbury and Ludlow. But the presence of castles is a reminder of past conflicts. This history makes the Welsh Borderlands an appropriate setting for one of the most rancorous, personally embittered and long-running battles in the history of geology - a border dispute over the boundary between the Cambrian and Silurian strata and the divisions of Earth Time that they represent.

Historically, it was a combination of rugged and inhospitable upland terrain in central and west Wales and the warlike nature of the Welsh clans that deterred even the Roman legions from venturing too far to the west. Nevertheless, the Romans did occupy areas of significant economic interest to them, such as Anglesey with its substantial copper deposits and the gold deposits near Lampeter in South Wales. Hill tops are often capped by the barely discernible remains of ring forts constructed by Celtic Welsh clans in Roman times. Major Roman garrisons were established at strategic points along the border, interconnected by typically straight Roman roads.

Several hundred years later, Offa, King of Mercia, even went to the extent of having a dyke (a ditch and wall) dug along the 150 and more miles of the border just to make an indelible mark as to what was Welsh and what was English territory. The passage of the dyke across the landscape of the Borderlands often coincides with the Roman roads. Beneath these relatively recent human-generated demarcations lies a much more ancient and fundamental change in the geological structure and strata between the two countries.

As we have seen, the Midland plains are founded on relatively soft and easily eroded sedimentary rocks belonging to the Triassic System. In places older Carboniferous deposits outcrop along the border from the Forest of Dean through Shropshire to Denbighshire in the north. To the west lie even older rocks, which in Smith's day were largely terra incognita. Those he was most familiar with were what he called the Red Rhab and Dunstone, also known as the Old Red Sandstone, below which lie a great thickness of older strata that he called the Killas Slate (an ancient Cornish miner's name for clay-slate). Unlike many of the gentleman geologists of the day, Smith had not benefited from the European geological education promulgated by the likes of Abraham Gottlob Werner. Instead, Smith was using the terminology of English quarry workers. To those of the Wernerian school, the Killas Slate strata belonged to the Transition Series.

The wide triangular swathe of the Red Rhab and Dunstone forms rounded hills and high moorlands south from Bridgnorth to the Severn Estuary and west to the Black Mountains and on to Pembrokeshire and the Irish Sea, and to the west lie the Killas Slate and other strata. This was the real unknown geological territory of southern Britain and presented a considerable challenge to a rising generation of young and ambitious men. Increasingly they were calling themselves geologists rather than natural philosophers. They were part of a growing band of practitioners of a new and exciting science in the 1820s and 1830s and they were intent on building their reputations and geological careers on 'Smithian' foundations.

While much of Smith's mapping of Jurassic and younger strata was reasonably accurate, the boundary lines of older strata needed extensive revision and subdivision with the introduction of new terminology. But there were relatively few 'virgin' tracts of rocks left that had not been mapped and western Wales was one of them. The others were southwest England and the huge region of the Highlands of Scotland with their immensely complex rocks and structures, which were still completely intractable for such a young science as geology.

The 'terra incognita' of the Transition Series

Although, strictly speaking, the Old Red Sandstone is next in the succession of older and older strata that we have been following, the revolutionary work that was carried out on the Old Red Sandstone was historically preceded by the mapping of Smith's Killas Slate and so on. Furthermore, the work that revolutionised the idea and understanding of the Old Red Sandstone was done by the same geologists who first conquered the Killas, and so I am going to follow the historical narrative and deal with the Killas, otherwise known as the Transition strata, first.

The theoretical understanding of the division of Earth Time at the turn of the eighteenth and nineteenth centuries was still greatly influenced by the German intellectual tradition. Rock strata appearing at the Earth's surface had been generally classified successively from younger to older as Tertiary, Secondary and Primitive strata. But this scheme was modified by Werner, who published a more advanced classification of rocks in 1787.

Above the Primitive rocks, Werner recognised what he called Transition rocks and these were primarily seen as chemical precipitates of a global ocean, although they included some stratified deposits, which were thought to have been produced by the erosion of Primitive rocks. As we have seen, Werner called his version of Secondary strata Flotz-Schichten or stratified rocks, and they were made up of fossiliferous sandstones, limestones, slates, coal and so on. Above these lay his Aufgeschwemmte-Gebirge or Alluvial strata (equivalent to the Tertiary deposits), formed by running water carrying eroded material from the land into the sea to form sands, peats and clays. In addition, there were volcanic products, such as ash and cinder beds, all of which could be fossiliferous. To Werner, his four subdivisions reflected the history of the formation of the Earth's crust.

As we have also seen, British geologists were deeply involved in the race to distinguish and name geological systems with recognisable 'packages' of strata, representing discrete periods of Earth Time. The Transition strata were generally not differentiated or mapped in any detail, but thought to be comprised of an upper sequence of limestone and shale below which lay Grauwackes or Graywackes (another Wernerian term for a kind of dark-coloured sandstone typically associated with the Transition strata). Even the 1820 compilation map by Greenough showed the older Transition, Grauwacke and Primary rocks of Wales, the Lake District, southwest of England and most of Scotland as 'terra incognita'. Indeed, Smith's nephew, John Phillips, wrote:

before the Summer of 1831 the whole field of the ancient rocks and fossils ... was unexplored but then arose two men ... Adam Sedgwick and Roderick Murchison and simultaneously [they] set to work to cultivate what had been left a desert.

As usual, the truth is somewhat more complex.

George Bellas Greenough, 1778-1855, member of parliament (1807-12), chemist and geologist, co-founder of the Geological Society of London and first President (1807-13, also 1818 and 1833). He compiled a geological map of England and Wales in 1820 that used some of Smith's work and undercut sales of Smith's own map.

The story generally told in books about the history of geological investigation in Britain is that the first systematic investigation of the strata of the Transition rocks of Wales was undertaken by Roderick Impey Murchison and Adam Sedgwick in the 1820s and 1830s. Murchison was of a Scottish family of landowners and minor aristocrats, but their land was relatively poor and provided little in the way of rents. Murchison knew from an early age that he had to make his own way in life and at the age of 15 went to the Military College at Great Marlow and joined a foot regiment in the British army as a subaltern. He may have got more than he bargained for, as he was immediately involved in the Peninsular War, taking part in Sir John Moore's famous retreat to Corunna over the Pyrenees in winter 1808-9. Thanks to some family influence he was then able to move to a more fashionable mounted regiment of Guards, but in 1815, on making a 'good' marriage at the age of 23 to Charlotte Hugonin, the daughter of a General, was able to resign his commission and 'do' the Grand Tour of Mediterranean Europe with his new wife.

The story has it that, although he was inclined to the usual gentlemanly pursuits of hunting and shooting, Charlotte encouraged him to do something more useful instead. Some versions of the story relate that thanks to a chance meeting at a dinner party, the eminent chemist Sir Humphrey Davey encouraged Murchison to take up geology. Maybe his well-connected wife prevailed on Sir Humphrey to do so. Anyway, Murchison attended geology lectures in London, which sufficiently enthused him to take up the hammer instead of the gun. He soon began independent researches into the geology of parts of Sussex, the northeast of Scotland and the Isle of Arran. He also made useful contacts with the rising stars of British geology and in 1828 geologised with Lyell in France and northern Italy. As a man of independent means he could afford to spend as much time as he wanted in the field, and with his social connections was able to make use of the hospitality of local gentry and aristocracy.

By contrast, Adam Sedgwick, son of the vicar and schoolteacher of the small village of Dent in Westmorland, had done well enough scholastically to gain a sizarship to Cambridge University. This meant that he had partly to pay his way by waiting on his fellow students; it was a well-trodden route for clever but poor students to gain degrees. Sedgwick did exceptionally well, was elected a fellow of Trinity College in 1810, was ordained at Norwich in 1817, and appointed to the Woodwardian chair of mineralogy in the university and a fellowship of the Geological Society in 1818, despite only having a limited knowledge of geology. He soon made up for this deficiency and was the first Woodwardian professor in many years to take his duties seriously. From 1822-4, Sedgwick made the first systematic geological survey of the Lake District (and in 1842 was asked by Wordsworth to write a geological introduction to his A Complete Guide to the Lakes), made the acquaintance of Murchison, and together they geologised in Scotland, Devonshire and Wales in the 1830s and began some detailed geological studies in the Austrian and Bavarian Alps. Sedgwick's time available for field work was constrained to the university vacations and even then he had ecclesiastical duties in Norwich to attend to.

In Wales, the two friends decided to see if they could make geological sense of the unknown terrain of the Transition strata. Sedgwick, with his mathematical skills and greater experience of unravelling structurally complex rocks with folds and faults, started his mapping from the oldest, Primary rocks of North Wales, with the intention of mapping his way south and east. Murchison was to work his way down from a known base line in younger rocks. He did his homework, assiduously picking the brains of any geological acquaintances who knew anything about the Grauwacke strata, such as Buckland and Conybeare. As a result of the advice he received, he started his investigation from the southern end of the Wye valley and worked his way northwards. He drew sketch sections down through the stratigraphic succession from the Old Red Sandstone strata into progressively older but richly fossiliferous strata, noting their characteristics and fossil content as he went. His young wife Charlotte went with him and, being a well-trained and accomplished sketcher of picturesque landscapes like so many genteel ladies of the time, she drew views of geologically interesting and significant features, which Murchison later used in his publications.

In retrospect, Murchison claimed that his mapping in Wales proceeded by 'Smithian' stratigraphic principles. Right from the start he certainly took care, whenever he could, to characterise his rock units by listing their fossil contents. But his palaeontological skills were limited and to begin with he was often basing his mapping on the physical appearance of the strata. For instance, he could differentiate between successions of shales or limestones, but since there is a considerable repetition of these strata types within the overall succession and their fossil content can to the inexpert eye seem similar, he sometimes got confused. In 1839 Murchison recognised an Upper Silurian made up from higher Ludlow strata and lower Wenlock Limestone, below which was his Lower Silurian comprised of the Caradoc Sandstone and Llandeilo Flags, and then below this lay Sedgwick's Cambrian strata.

Roderick Murchison's wife Charlotte was, like so many of her contemporaries, accomplished at sketching landscapes and provided illustrations such as this view of the Carneddau Hills from the Wye Valley for her husband's 1839 book on The Silurian System.

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Freehand Sketching An Introduction

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