Paleozoic To Archean

William Smith made no attempt to deal with the oldest rocks of North Wales at the northwest end of his section. Indeed, until the latter part of the eighteenth century such mountainous terrains of the British Isles were still almost completely unknown territory and generally regarded as not only useless and inhospitable, but also uncivilised, undesirable and generally to be avoided. However, attitudes were beginning to change as artists and poets, often from urban middle-class backgrounds, began to discover untramelled nature away from the fields, pastures and parks cultivated by their fellow humans and look to the savage and rugged beauty of rocky mountains, narrow passes and deep ravines, with their sense of the unknown, danger, forboding and deep antiquity.

Mountains presented an exciting challenge to the jaded senses of urbane and refined society, as portrayed by Jane Austen. Although the mountain and rocks of North Wales did not have the grandeur of the Alps, which so many of the well-to-do admired as they crossed into Italy on the Grand Tour, they did attract some very capable artists such as Richard Wilson, Cornelius Varley, John Sell Cotman and David Cox, whose paintings of their wildness attracted crowds of admirers when they were first displayed in the fashionable salons of London. Some of these North Wales localities were among the first tourist attractions of nineteenth-century Britain, but the intricacies of their geology were far beyond the understanding or interest of even the literate middle classes.

Even among the geologically minded naturalists of the day, understanding of the nature and formation of mountains was still exceedingly primitive at the end of the eighteenth century. Werner's standard succession of the oldest rocks was claimed to reflect the order in which they appeared as they crystallised out of the primeval ocean. The theory persisted in Britain into the first decades of the nineteenth century, largely due to the influential teaching of Robert Jameson in Edinburgh and Richard Kirwan in Ireland, who was a vehement opponent of Hutton.

Richard Kirwan, 1733-1812, Irish chemist and natural philosopher who was firstly a Jesuit novitiate (1754), then called to the Irish bar (1766), studied science in London, returned to Dublin as a professor of mineralogy (1787), published Elements of Mineralogy (1784), President of the Royal Irish Academy (1799)-

By the 1820s the sequence had been elaborated to 15 or so separate rock types, ranging from the youngest 'Newer serpentine' and 'Sienite' down to 'Primitive limestone', 'Primitive trap', Older porphyry', Clay-slate', 'Topaz-rock', Mica-slate' and 'Gneiss' to the oldest and most primitive 'Granite'. All of these were seen to have some sort of stratification or foliation that was thought to be primary, except for granite. The latter with its large crystals and randomly formed, non-laminar masses had to be the oldest. Buckland was still using 'Clay-slate', Mica-slate', 'Gneiss' and 'Granite' for his illustrated succession of Primary Stratified Rocks in his Bridgwater Treatise volume on Geology and Mineralogy published in 1836. Early nineteenth-century attempts to unravel the geology of the oldest rocks of North Wales, such as those by Sedgwick and by Henslow, Darwin's Cambridge mentor and Professor of Botany, were doomed to failure.

Rev. John Stevens Henslow, 1796-1861, Cambridge botanist, first appointed professor of mineralogy (1822-7) then botany (1827-61), vicar of Hitcham, Suffolk (from 1839). Recommended Darwin as a naturalist to accompany Fitzroy on HMS Beagle.

With the development of the Cambrian/Silurian controversy and Murchison's claim that the origin of life was to be found in his 'protozoic' Silurian strata, questions arose about the nature and naming of pre-Silurian rocks. Murchison tended to refer to them as 'Azoic' or 'crystalline', but by the 1850s William Logan had found some curious organic-looking structures in limestones interbedded with ancient gneiss in Canada (see p. 266). Logan discovered that much of that country (the so-called Canadian Shield) is comprised of an extensive complex of crystalline metamorphic rocks - gneiss and schist, which form a basement to the oldest fossiliferous rocks. When William Dawson proclaimed Logan's organic-looking structures to be the genuine remains of primitive life, the options were either that life had originated in times represented by pre-Silurian/Cambrian rocks, or that the rocks were themselves also Silurian in age. Logan tried to resolve the question by differentiating older Laurentian gneisses from a younger Huronian series of metamorphosed sedimentary rocks that contained the fossils.

Sir John William Dawson, 1820-99, Canadian-born geologist educated in Edinburgh. On return to Canada as schools superintendent in Nova Scotia (1850), geologically surveyed the region, became professor of geology at McGill (1855-93), first president of the Royal Society of Canada, knighted in 1884.

Similar initial attempts were made to subdivide older rocks in Europe, but there was a growing awareness from studies of the European Alps that not all metamorphic rocks were so ancient. Therefore it was possible that the process of metamorphism could happen at any time in the geological past. If that were true then, in the absence of fossils, any attempt to give a relative age to such rocks was problematic. If they were overlaid by 'datable' fossiliferous strata, the best that could be said was that they predated those strata. By 1862 the name Precambrian was formally defined by Joseph Beete Jukes for all those rocks older than the Cambrian and has been in general use ever since. At the time, nobody had the remotest idea how much of Earth Time was represented by the Precambrian.

Joseph Beete Jukes, 1811-69, Cambridge-educated geologist and pupil of Sedgwick's, worked on the geological survey of Newfoundland (1839-40), naturalist on the voyage of HMS Fly (1842-6), director of the Irish Geological Survey (1850-9) and worked on the Royal Commission on coalfields, 1866.

In 1877 British Precambrian rocks were subdivided into four by Henry Hicks, namely the Lewisian in Scotland and the Dimetian, Arvonian and Pebidian in Wales. Similar attempts were made elsewhere in Europe and in North America (see p. 249) and the gradual process of trying to make some geological sense of the vast stretch of Precambrian time had begun.

As the understanding of metamorphism and structural geology improved, it became possible to attempt detailed analyses of complex Precambrian rocks. In Britain there were geologists such as Edward Greenly who devoted many years of their professional lives to mapping the Precambrian geology of particular regions. Greenly tried to decipher what had happened geologically on Anglesey and in 1919 published a two-volume memoir in which he drew comparisons with structures and metamorphic rocks found in Anglesey and the Lewisian rocks of the Highlands of Scotland. Even today, the structure and relative ages

Henry Hicks, 1837-99, Welsh physician and geologist who worked with Salter on the palaeontology of Cambrian rocks in Wales (1860s), the Precambrian of Anglesey, Devonian and Quaternary.

Edward Greenly, 1861-1951, British geologist who devoted many years to a geological survey of the Precambrian rocks of Anglesey, published Methods of Geological Surveying (1930) with Howel Williams.

of the rocks are far from clear, and recently comparisons have been made with rocks found in the southeast of Ireland and with rocks of Precambrian age in Newfoundland.

As we have seen, the lowest and oldest strata of Sedgwick's Cambrian strata in North Wales consist of gritty sandstones and conglomerates, which are interpreted as shallow-water deposits formed as the Cambrian seas flooded over (transgressed, as geologists call it) an eroded and subdued landscape developed on more ancient rocks. The Cambrian strata are themselves strongly fractured and cleaved and somewhat metamorphosed, so that if there were any fossils in them originally, they would have been destroyed. Consequently, their age is not really known and it is by correlation with similar deposits elsewhere that they have been assigned to the early Cambrian.

On the island of Anglesey and in the Llyn Peninsula of North Wales there are some 7000 m of older, unfossiliferous sedimentary strata with interbedded volcanic rocks, all of which seem to have been metamorphosed and deformed to a greater degree than the younger Cambrian rocks elsewhere in Wales. They include an extraordinary rock unit called the Gwna Melange, which contains blocks of older rocks ranging in size from a few millimetres to a kilometre or so in diameter. Edward Greenly thought that it had been formed by tectonic processes, but such deposits (called olistrostomes) are now known to be generated by deposition within actively subsiding and deforming ocean trenches associated with subduction.

The relative age and subdivision of these rocks are still matters of controversy and the nature of the boundary between them and undoubted Cambrian strata is still unclear. Some of the strata could also be of early Cambrian age, a possibility that has been strongly supported by the discovery of marine microfossils in some of the fine-grained sedimentary strata. Below the bedded series of rocks on Anglesey lies a considerable thickness of more highly metamorphosed and

Presently recognised major divisions of Earth Time with their 'authorship'

Holocene (Gervais, 1867) Pleistocene (Lyell, 1839)

Quaternary (Desnoyers, 1839)

Pliocene (Lyell, 1833) Miocene (Lyell, 1833) Oligocene (Beyrich, 1854) Eocene (Lyell, 1833) Palaeocene (Schimper, 1874)

Cainozoic (Phillips, 1840)

Cretaceous (D'Halloy, 1822) Jurassic (von Buch, 1839) Triassic (von Alberti, 1841)

Mesozoic (Phillips, 1840)

Permian (Murchison, 1841)

Pennsylvanian (Williams, 1891) Mississippian (Williams, 1891) Carboniferous (D'Halloy, 1808; Conybeare & Phillips, 1822) Devonian (Murchison & Sedgwick, 1839) Silurian (Murchison, 1835) Ordovician (Lapworth, 1879) Cambrian (Murchison & Sedgwick, 1835)

Palaeozoic (Sedgwick, 1838; Phillips, 1841)

Precambrian (Jukes, 1862)

presumably more ancient gneisses that Greenly compared with the Lewisian gneisses of Scotland, but their status is also far from clear. The Precambrian story will be continued on the other side of the Atlantic in the depths of the Grand Canyon.

0 0

Post a comment