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Britain’s rocky roots

December 12, 2016 Leave a comment

Imagine that the oldest rocks in Britain, the gneisses of Scotland’s far north-west, were formed 12 hours ago (rather than 2.7 billion years). Then adjust all the subsequent timings so that the rocks of this island come together in the course of a day.

It’s a slow start. The Torridian sandstones, which form the mountains which rise from the gneiss, were created a full six hours later. Not much else can be said about those intervening six hours, except that it took that long for the gneisses to make their way to the surface from 30km underground. The oldest rocks in England and Wales – which form the Malverns, parts of the Long Mynd, the area around St David’s, and other fragmentary but compelling patches of landscape – came into being an hour after that.

That leaves about three hours, and almost all of Britain has yet to appear. Nevertheless these rocks are important: they make memorable places; are a glimpse of the kind of bedrock that lies buried deep beneath the entire island; and are reminders of that profound strangeness: a world where complex life barely exists.

The big change comes in a period of about 40 minutes either side of two hours from now. In a continental collision somewhere in the Antarctic, two formerly separate gatherings of ancient rocks, parts of which we can call ‘Scotland’ and ‘England’, are conjoined. Himalayan-scaled mountains go up as vast quantities of sediment fall, buckle and fold; fault-lines such as the Great Glen are set up, marking the landscape to this day. Volcanoes explode. The chief mountain ranges of Britain are the battered remnants of this event: the Highlands, the Lakes, Snowdonia. It’s all also broadly contemporary with the first great explosion of complex life: fish, for example, appear during this period.

These heights then erode for 20 minutes or so, reducing in scale as they do so (indeed they have been getting smaller ever since: what we witness are barely stumps of the originals). Their sediments lay down the Old Red Sandstone of Herefordshire, the Orkneys, and elsewhere; many of the rocks of south-west England come into being now, too, side-effects of a further continental collision happening off-screen to the south (which also creates a few more volcanic events).

Then a complex era in which our patch of the planet moves, over just 10 minutes, from desert to oceanic to island-edge and all stages in between. This creates the sandstones and limestones of the Pennines (England’s largest area of highland), and much of south Wales and lowland Scotland. Many of the sediments of which all this is created are themselves, in effect, fossils. Coral reefs become limestone country, ready to erode into peaks and caves; swampy forests become coalfields. Life, then is by now commonplace, and much of the highland armature of the island has been brought into being.

We have less than an hour and a quarter to go, and the rocks that dominate the soft south-east third of the island have yet to develop. Not that it’s an island yet: indeed, we are now locked in the middle of a supercontinent. We have been moving gradually north, and are somewhere around the equator. In a vast desert, the New Red Sandstone that dominates the English Midlands (and which stretches its arms far to the north either side of the highland) is laid down; then, beneath warm oceans, the yellow Jurassic limestone and the white chalk. The New Red Sandstone, Jurassic limestone and chalk lie on top of each other, and cover in turn the older, harder rocks beneath; each, today, begins to the south and east of its predecessor, like the sheets of a well-made bed. Each takes 20 minutes or so to be laid down; during this hour-long process, the age of the dinosaurs comes and goes.

Most of our island now exists. And every rock, as it appears on the surface, begins to erode and reduce, reconfiguring the appearance of the landscape and creating the potential to make new rocks.

By the last quarter of an hour, our layered jigsaw of stones has reached a temperate part of the northern hemisphere, is located on the edge of the landmass that is now Europe, and has an ocean widening to the west, as what is now America breaks free and moves away. Massive eruptions accompany this process, leaving behind them the rocks which form the sharpest peaks on such Hebridean islands as Skye and Mull, and some of the most significant other volcanically-derived bits and pieces in the British landscape.

Only the softest and most friable of stones, such as those which make up the cliffs of the East Anglian coast, or the clays on which London lies, are still forming. Indeed the whole assemblage has become, through millenia of erosion, layering and addition, a ghostly precursor of the modern pattern of upland and lowland which characterises this land; even many modern rivers have rough predecessors. Though the sea is both rather mobile and not at all where we might expect it (a proto-Thames, for example, flows scores of miles north of its current course and drains into a mighty river located somewhere under today’s North Sea), a shadow, counterfactual Britain is discernible.

The ice comes in the final minute of our story. Until just a fraction of a second ago, it has been polishing and grinding the landscape into its current shape. As it recedes, rivers find their current courses, the sea its approximate present level. Britain comes into quite sudden definition as the largest island in an archipelago on the north-west coast of Europe. Perhaps around a twentieth of a second ago, man settles for good here for the first time, starts fiddling around with rocks, creating tools, clearing forests, making fields, building houses and cities, digging quarries and mines; in short, transforming in the blink of an eye the surface appearance of the entire island. We reach the present, hurtling into the future. And this whole story only covers half the history of the planet in which it took place.

As an afterthought, these major formations can be covered, roughly in chronological order, in a 12-hour drive from the far north-west to the far south-east of the island. To get this drive to line up with the amount of time involved in creating the rocks in the first place is a tall order: generalising hugely, it involves driving at 7 miles an hour or so for the first 70 miles (Cape Wrath to Ullapool; the formation goes on another 70 miles or so, but that’s splitting hairs); to then cover 190 miles in 40 minutes, that is, to drive at nearly 300 miles an hour from Ullapool to the southern edge of Loch Lomond, cutting over to the northern portion of the Midland Valley for 20 minutes before skipping the rest of Scotland –thus exemplifying the interrupted, unconformable nature of geology – before resuming to cover the length of the English Pennines, from a point somewhere halfway between Carlisle and Berwick down to Derby (not for the first time, no road will do this: you have to go either side) — about 180 miles — in 10 minutes. One then has to go at 180 miles an hour (during which one passes through at least three major rock formations) until one is almost at the south coast. Having arrived at, say, Cosham – on the mainland side of Porsmouth – one has 15 minutes or so to cover the Quartenary’s portion of our hour and get to the seafront.  Given Portsmouth traffic, this may prove the hardest bit of all. Anyway, I guess the post-ice age/human presence portion is roughly equivalent to a walk along the rocky beach, but at that point my computational skills collapse. Except to comment that the way our island looks, including the fact that it *is* in island – let alone all the amazing architecture that has been built from its geology, from Durham cathedral to Buckingham palace – is about as temporary and impermanent as the coast at low tide.

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