Article 61

Further To The Dethronement Of Belief By Knowledge.

With my Article 60, having reviewed the geological knowledge contained in the Introduction to Peter Toghill’s Book, I now relate this Section of Articles to his successive Chapter headings.

In his Chapter 1: Britain During The Precambrian Period, Peter Toghill observes that Britain’s oldest rocks are difficult to unravel because of the amount of deformation and metamorphism which they have undergone; that rocks of this Period were initially thought to be devoid of fossils and the Precambrian/Cambrian boundary was initially drawn at the base of the first rock sequences to contain fossils of abundant and varied life forms. However, he notes that we now have rare but widespread knowledge of marine plants and soft-bodied animals within the Precambrian strata, and we may eventually find therein the ancestors of the abundant hard-shelled life-forms of the subsequent Cambrian period; that the Precambrian stretches over 4000 million years from the origin of the Earth’s crust at 4600 Ma (million years ago) to the start of the Cambrian at 544 Ma; that the rarity of Precambrian fossils means that we cannot divide up this huge period as we can the Cambrian and later periods; that there is, however, a division into an older almost wholly metamorphic sequence called the Archaean (older than 2500 Ma) and a younger Proterozoic subdivision which contains a good deal of sedimentary rock and some marine fossils; and that the metamorphic Archaean rocks form the basement to all continents; but that they became visible on the surface where erosion and earth movement resulted in their exposure

He recalls that Britain’s oldest rocks are in north-west Scotland, from the south-eastern corner of Skye to Cape Wrath on the mainland; that they also occur extensively on the outer Hebridean islands of Harris and Lewis and as outcrops on the inner islands of Tiree, Coll, Iona and Islay; that Some of these Lewisian gneisses are Cambrian and others are Proterozoic; that in 1994, the age of one of these mainland gneisses was measured as of 3300 Ma; but that later work suggests that the oldest gneisses are of 2900–2750 Ma. Again, he recalls that the Archaean rocks found in Canada have been dated at 4000 Ma, the oldest rocks yet found. Again, he suggests that it is worth going to north-west Scotland to stand on and touch its rocks and to consider what the original basalt would have been like before it was metamorphosed to gneiss at around 3000 Ma. He recalls that geologists now consider that at the time when continents were being built, the Earth’s atmosphere contained no oxygen, at which point, I note that the earliest living things must have been photosynthesising single-cell plant forms. From this point onwards, I want my readers to know that unless I note otherwise, I am gratefully quoting Peter Toghill’s book: I am not a geologist. I am a physical chemist quoting a geologist to exemplify the benefits of knowledge over belief and counter-belief, unless such beliefs are hypotheses for reality-evaluation to further knowledge, either positive or negative, as in my campaign for knowledge to replace belief wherever possible.

The first and oldest unconformity which we come across in Britain is that separating the Lewisian gneiss in north-west Scotland from the Torridon Sandstone Group. The sandstones are of late pre-Cambrian (Proterozoic) age and have been dated at around 1000-700 Ma. This red sandstone appears to consist of river and lake deposits and indicates an arid climate close to the equator while measurement of their ancient magnetic fields indicates that they were deposited at 15 degrees North. These remarkable mountains of north-west Scotland, including Suilven and Canisp are formed of great terraces of these sandstones and from their slopes one is really looking at an exhumed pre-Cambrian landscape, which was inundated by the sea at the start of the Cambrian period at around 544 Ma and here we find our next unconformity with the white Cambrian sandstones, the Eribol Quartzite, resting on eroded Torridon Sandstone around loch Assynt. Again, these Cambrian Quartzites are followed by sandstones which have trilobites in them, the first abundant marine organism, and these are followed by tropical limestones (Durness limestones) which continue into the Ordovician. Thus, this plate-tectonic evidence indicates that this whole area of north-west Scotland was part of the eastern seaboard of what is now North America, an area which geologists call Laurentia, while it was south of the equator, while a new ocean, the Iapetus Ocean, was opening up under the influence of its developing mid-ocean ridge, and while what was to become the rest of the British Isles (including Eira) was moving away from it, while attached to the other side of the then opening ocean towards a subduction zone beyond 60 degrees South.

Scotland, east of the then opening Iapetus Ocean consists of the Northern Highlands, an area between the Moine Thrust and the Great Glen Fault, which is occupied by a 10 km thick rock sequence of Precambrian Sandstones and shales metamorphosed to schists and other rocks called the Moine Schists or the Moine Supergroup. These are intensely deformed, with slices of Lewisian Gneiss within them in places. These have been dated at between 1200 and 870 Ma and were metamorphosed around 1000 Ma, at 800 Ma and again around 460 Ma, and these strata are intensely folded. The sediments from which the Moine Schists have been formed may in places be lateral equivalents of the Torridon Sandstone Group. Folding and metamorphism depend on subduction zones and seafloor spreading, but here we have little evidence of ancient oceans, only of sediments formed in relatively shallow seas. However, most geologists look for comparable sediments in North America and Greenland (Laurentia) rather than further south in Scotland. South of this area, we cross the Great Glen Fault and enter the Grampian Highlands and encounter large tracts of mainly metamorphic rocks, this time called the Dalriadian Schists or the Dalriadian Supergroup. Towards the top of this sequence are found rare Cambrian and Ordovician fossils in a sequence which in part is equivalent to the Cambrian shallow water sequences of the far north-west.

This Supergroup comprises a very thick (up to 24 km) sequence of originally varied sedimentary rocks now largely metamorphosed to slates and schists. However, the grade of metamorphism decreases south towards the Highland Boundary Fault and towards the top of the sequence. Again, however, the effects of two later episodes of mountain building make it difficult to understand all local sequences. These rocks have been shown to follow on top of Moine Schists around the great Glen Fault, and are not older than 850 Ma. They accumulated on a shallow shelf, with stromatolite (algal) limestones now occurring on Islay. This shelf migrated north-west by the early Cambrian at 540 Ma, while the overlying Argyle Group contains at its base a remarkable sequence of beds indicating a late Precambrian glaciation – the famous Port Askaig Tillite (a tillite is the name given to an ancient glacial deposit produced by an advancing glacier and left behind when it retreats (otherwise known as glacial moraine). This glacial sequence contains evidence for over forty separate ice advances. Even more remarkable is the magnetic evidence that this glaciation occurred within 10-15 degrees of the equator within a sequence of shallow water calcium-magnesium carbonates. This Precambrian glaciation, dated around 600 Ma may be unique in that it affected all latitudes: there is evidence for it from other parts of the world. The boulder clays, thickest on Islay at 75 m, contain large boulders and appear to have been formed by a grounded ice sheet in a shallow sea. Similar deposits in Scandinavia show evidence of movement from the south, as do the Scottish deposits, but as we shall see, they were both connected before being separated by a deep and widening Sea.

Above the glacial deposits, we find a further 20 km thickness of Dalradian rocks within Argyle (9 km) and Southern Highland Groups (11 km) many of which are metamorphosed. These later sediments are very thick and show evidence of initially being formed on a rapidly subsiding shelf. They include sandstones like the Jura Quartzite, which is 5000 m thick, as well as thin limestones. Later on, the deposits show evidence of a much deeper ocean basin. A famous granite at Ben Vuirich intrudes the Argyle Group and is dated at 590 Ma, so we know the tillites are older than this, while recent work around the tillites has provided evidence for the earliest metazoan animals, possibly marine worms, while within 8 km above the tillites we find a sequence of submarine basalt lavas, the Tayvallich Volcanics dated at 594 Ma. These lavas occur above the Tayvallich limestone on the Argyle coast south of Oban and may be associated with seafloor spreading and a new ocean forming between Scotland and Scandinavia. This subsiding ocean basin then received a further 11 km of sediment in the later Dalradian Group, the highest limestone beds of which contain Middle Cambrian trilobites, so we know that the widening of this ocean basin went on into the Cambrian Period. The opening and closing of this Iapetus Ocean belongs to the Cambrian, Ordovician and Silurian Periods.

However, the Pre-Cambrian rocks in England and Wales only cover small areas compared to Scotland and rather than including Peter Toghill’s account of them here, I simply refer to the Summary with which he concludes the first Chapter of the book to which I am referring. Thus, he summarises by saying that we have now completed our look at Britain’s oldest rocks. In Scotland we have seen huge areas of Precambrian rocks in which the story has with difficulty been unravelled because of the amount deformation and metamorphism; that nonetheless it has been possible to see the development of sedimentary basins on the edge of the Laurentian supercontinent; that these basins were situated in the tropics and yet were affected by a glacial episode at 600 Ma; that during the late Precambrian the amount of sediment being deposited in the ocean basins was very great; that this continued into the Cambrian; that the basins were then subjected to the Grampian orogeny and folded up and metamorphosed during the early Ordovician; but that we cannot see the southern limit of this metamorphosed and folded area, as it is buried under younger rocks to the south but we can see the north-western edge of the fold mountains where the Moine Thrust pushes the metamorphic area north-west over the unaltered Torridonian and Cambrian of the far north-west; that in England and Wales we see small areas (inliers) of Precambrian rocks peeping out from the younger rocks where the story is of a poorly formed metamorphic basement overlain with young Precambrian sediments and volcanic rocks formed within an ocean basin with volcanic island arcs deformed by the Cadomian orogeny at the end of the Precambrian. 5/7/21.

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