by Ted Nield
But how did the gypsum get there in the first place? To find that out we need to treat Alice’s rabbit hole as a time tunnel to Wegener’s lost supercontinent, just as it is beginning to break up.
In the mind a map
Leaving behind Field View, Mrs Sherwood-Britton’s old house, you return to the main road at Ripon’s medieval North Bridge. Pausing to admire the piers of the new A61 Ripon bypass, with their invisible big feet, you turn through the city centre and head out on the B6265, due west, towards the Pennines: the backbone of England. Passing Royal Studeley, its deer park and Fountains Abbey World Heritage Site on your left, you take the turning to High Grantley and make your way up the rich, gently climbing farmland to High Skelding and Dallow Moor.
Quite suddenly, after the rich farms, their fertile fields and ancient hedgerows dotted with oak trees, the landscape begins to change. Drystone walls replace the hedges, and then themselves vanish as you pass on to the open moor underlain by Millstone Grit, Carboniferous-age rocks that were already deposited, hardened and folded into mountains, as the supercontinent of Pangaea assembled. They are humbler now, of course, after 250 million years of erosion; but as you walk up the heathery slopes to stand on a rough lump of Grit or in the lee of a sheep shelter and look down across the lowlands to Ripon, you are climbing the exhumed topography of Pangaea.
Close your eyes. You hear the wind over the heather, gorse bushes and hawthorns. Larks are rising. Mournful, far-off cries of birds of prey drift across the hillside. It is time to follow the White Rabbit, and imagine his pocket watch racing backwards 250 million years.
You are now just two million years or so into what geologists have named (after the Chinese locality where its deposits are best displayed) the Wuchiapingian Age of the Late Permian. You, and those barren grey mountains of Millstone Grit behind you, whose roots will one day lie exposed to the boots of fell walkers, now sit about twenty degrees north of the Equator; the same latitude as Port Sudan, Timbuktu and Santiago de Cuba today. Although the Sun is slightly less luminous, it is searingly hot, perhaps in the upper forties. From the bare rock at your feet, you take in the slopes of brown, varnished boulders stretching to the plains, a sea of brownish dunes, up to fifty metres tall. Not everywhere down there is covered in sand; in some places rock desert pokes through, and in others, more low-lying, you can see greyish white: the salty fine mud of a desert playa. Towards the horizon its dirty white merges into the mirage. Towering greenish dust devils suck at the dry mud and, driven by the merciless Sun, carry it high into a sky where no bird has ever flown.
Behind you the Pennine range separates you from similar lowlands to the west, but you would have to cross another 4000 kilometres of mountain and desert – nearly all of the future North American continent – before you would come to a coast. But what a coast! This longitude-paralleling shore stretches almost from pole to pole.
Eastward you could travel through all of Europe, across the newly formed Ural Mountains, and across much of modern Siberia beyond, before you would meet sea again. To the south (following the ‘N’ on your compass because this is a time of magnetic reversal) you could walk all the way to the South Pole. This period, when the north magnetic sits at the south geographic pole, will last for another two million years before flipping back again over the geological instant of a few thousand years. But by that time the eastward view below you (over what will one day be the Vale of York) will have changed beyond recognition.
You feel as though marooned on a raft in mid-ocean, lost in the remotest heart of this seemingly endless expanse of parched land. But though dry, it is not completely lifeless. In the folds of the higher ground, or nestling at the foot of rocks where water comes nearer to the surface or seasonal rainwater collects perhaps once a decade, shrubby horsetail-like plants grow in clumps, relying on their tough, deep rhizomes to find what water they can. Another shrub, Peltaspermum, and in rare places some conifers related to pines and the gingko, provide sparse shade. The living is about to get a bit easier for some of these survivors. Still others will be overwhelmed.
Since the beginning of the Permian Period forty million years before, the supercontinent on which you are sitting has slowly drifted about fifteen degrees north. The time of the greatest dying in the history of life, the most severe mass extinction in the geological record, is almost upon the Earth. In the next few million years 90 per cent of all species now living will become extinct. After Pangaea, nothing will ever be the same again.
You might have noticed (had you really been trekking up these same gritstone slopes in the Late Permian) that you had become a little more out of breath than usual, as though you had suddenly become a lot less fit. That is because the air is thin. The atmosphere of the modern world contains about 21 per cent oxygen. About 300 million years ago, before the Permian began, with the Carboniferous coal forests pumping out oxygen from their rampant photosynthesis, it had been higher still, at about 30 per cent. But the atmosphere’s oxygen has been tailing off as the great coal forests dwindled. In the late Permian it stands at about 16 per cent. Breathing at sea level has become like breathing at 3000 metres in the modern world; but by the time global oxygen levels bottom out at 12 per cent, as in the next few million years they will, living at sea level will be like living at almost 6000 metres today – higher than any permanent human settlement in the modern world. Little wonder, then, that the four-legged land animals stalking the Permian landscape were having a bad time. Of the forty-eight families of such beasts recognized among the rare fossils found in land sediments, thirty-six had died out by the end of the Permian. Lack of oxygen was one factor among many that drove them to their doom.
The whole globe has also been slowly, fitfully getting warmer. Carbon dioxide and other greenhouse gases have been building up in the air, making life even more difficult. As temperature rises, animals’ metabolic rates also rise, further increasing their need for oxygen. And although at this time it may not quite have begun as you sit on the Pennine slopes, far over those distant Ural mountains, in what will one day be northern Siberia, the Earth is about to split asunder in a catastrophe surpassing any biblical horror.
Over a period lasting as little (to a geologist) as 500,000 years, and almost exactly coincident with the disappearance of most living species 250 million years ago, massive eruptions will spew between two and three million cubic kilometres of lava on to the Earth’s surface in our planet’s biggest-ever series of volcanic eruptions. Today those lavas, known as the Siberian Traps, cover 350,000 square kilometres and are nearly four kilometres thick in places. Significantly for life on Earth, along with that molten rock also came (according to one estimate) 10,000 billion tonnes of carbon, most of which ended up in the atmosphere.
Carbon dioxide will not be the only gas evolved. Sulphur dioxide will also pollute the air and acid rain will fall everywhere. The sea’s plankton, bottom link of the oceanic food chain, will be decimated. Rising temperatures will also affect the sea bed. Billions of tonnes of methane, trapped in cold, shelf-edge sediments, will suddenly become unstable and rise catastrophically to the surface, liberating even more carbon into the air than the Siberian volcanoes. Methane is the most powerful greenhouse gas of all, and thus the spiral of environmental breakdown will career towards the greatest extinction in Earth history.
Although there is no ice at the North Pole (and, like today, no land either), the South Pole has been located within Pangaea’s southern half, Gondwanaland, for millions of years. There a massive icecap has eroded huge amounts of ancient rock and pushed it out over much of what is today India, South America, Antarctica and Australia; but it has now dwindled almost to nothing. Mud and boulders the size of men have been dumped everywhere as the ice fell back, like the abandoned weapons of a retreating army, for the Blanford brothers to find at Talchir. Water, locked in the ice sheets for millions of years, has poured into the global ocean, Panthalassa, which has begun a fitful but inexorable rise.
The formation of Pangaea
itself, coupled with the removal of carbon dioxide from the air by land plants, triggered the great Gondwana ice age. Although this lasted for many millions of years and the centres of glaciation migrated as Gondwanaland shifted relative to the South Pole, the decisive moment came when the northern continental mass joined Gondwanaland at the close of the Devonian Period 355 million years ago. This closed off the equatorial current that helped distribute heat about the globe and Pangaea was born in an ‘icehouse’ world.
But at the end of the Permian all this is changing. The Earth system is flipping from icehouse to greenhouse mode. The increased volcanic activity associated with the break-up of Pangaea, and the eruption of the Siberian Traps, will both pump greenhouse gases into the atmosphere. The melting of the glaciers is already increasing the Earth’s absorption of the Sun’s heat. As the submarine spreading ridges become more active, they are swelling up and pushing the oceans, already full of glacial meltwater, on to the continental shelves. This in turn is further raising the heat-absorbing capacity of the planet because water absorbs and retains heat better than land.
The Earth’s climate will remain in its new greenhouse state for much of the next 220 million years, until quite close to our own time, as life gradually recovers from the end-Permian disaster, the dinosaurs come and go, birds fill the sky and the supercontinent of Pangaea slowly dissolves, fragmenting into today’s map.
As you sit on the proto-Pennines, something of those tectonic processes tearing Pangaea apart is at work below you, under the parched desert lowlands to the east. The seemingly endless plain is feeling the tension as Pangaea’s northern half, Laurasia, begins to unzip: a process that will eventually form the Atlantic Ocean. The floor of the plain is subsiding, falling like a keystone into a widening rift, and desert sediment is tumbling in, piling up into layers of porous rock that will one day act as reservoirs for North Sea oil and gas. As the supercontinent rends itself, the tension extends northwards along a relatively narrow belt between the ancient rocks of Greenland and Norway, now lying cheek by jowl.
There in the distant north, perhaps a week or so before your visit, the inevitable has finally happened. Thanks to that subsidence (despite the inrush of sediment that is doing its best to fill the vacuum), the desert plain over which you gaze lies up to 250 metres below sea level: an ancient Death Valley on a vast scale, stretching from here far into eastern Europe. That entire desert basin is about to become sea, and the whole flooding process will take just a few months. These two events, the flooding of the North Sea basin and the eruptions of the Siberian Traps, took place at about the same time in Earth history, and show how misleading the strict uniformitarianism of Charles Lyell can be. Our knowledge of what is normal behaviour for the Earth is extremely limited. Human beings have not existed on the Earth long enough to have witnessed the eruption of a Large Igneous Province. Nor has our species ever seen a major inundation like that about to unfold at your feet; though neither event is that uncommon in the long history of the Earth.
A wild surmise
Take an example from more recent geological time. The modern Mediterranean would not exist if it were not connected to the global ocean via the Strait of Gibraltar, because not enough water flows off the land that surrounds it to outstrip the process of evaporation. But the Strait is shallow, and about six million years ago, during the Miocene Period, global sea levels fell so much that the connection was broken. The Mediterranean dried out, leaving a vast desert basin that was only retaken by the sea 900,000 years later (still millions of years before modern humans were around). If you ever wondered why there are so many deep gorges in the South of France, this is the reason. Starting from their new bases (the bottom of the desiccated Mediterranean), these rivers eroded back rapidly along their courses, cutting deep, slot-like canyons like cheese wires slicing through a slab of Cantal.
Long before this became fact, H. G. Wells was studying geology at London University under his uninspiring teacher John Wesley Judd (‘washing his hands in invisible water as he talked’). The science-fiction pioneer learnt about geological speculations that the Mediterranean had once been dry, which were then being bandied around as explanations for some odd distributions of plants and those mysteriously deep gorges. In 1921 Wells incorporated the (then) unsubstantiated theory in a tale he published in the April issue of Storyteller magazine about an encounter between modern humans and Neanderthals. It was called The Grisly Folk.
‘It was in those days before the ocean waters broke into the Mediterranean that the swallows and a multitude of other birds acquired the habit of coming north, a habit that nowadays impels them to brave the passage of the perilous seas that flow over and hide the lost secrets of the ancient Mediterranean valleys.’
Now, Wells’s suggested timing for the Mediterranean’s big flood was awry by several million years, and as a theory the ‘dry Med’ remained unproven until 1970. On 28 August the Deep Sea Drilling Project research vessel Glomar Challenger was poking a hole in the western Mediterranean floor, south of the Balearic Islands. They were drilling in almost 3000 metres of water at the time, so the geologists on board were greatly surprised when they picked, from between the teeth of their drill bit, chunks not of gypsum exactly but of the anhydrous calcium sulphate mineral anhydrite. You find anhydrite only where there has been evaporation, and you certainly don’t find it on the bottom of the ocean – unless of course the ocean had once dried up and then been reclaimed by a catastrophic flood.
Something very like that flood is now unfolding below you as you perch on the Permian Pennines. Geologists continue to debate the relative role of local subsidence versus global sea-level rise, but the evidence in the rocks for the speed of the inundation is dramatic and unequivocal.
What first signs of the advancing tide might you sense from your vantage point? Some tang on the prevailing north wind of this desert basin perhaps? A smell like that of rain on parched city streets? Distant thunder as an unseasonal line of thunderheads advances, formed as water vapour convects violently off the desert and climbs miles high into the atmosphere? Or perhaps a sudden increase in the abundance of small reptiles, fleeing before the advancing menace, planting their three-toed tracks in the sands?
Such events as these would leave few or no conclusive traces; but still we can tell the flood was sudden. The evidence lies in the remains of those sand dunes, which today survive as some of the topmost sandstone gas reservoirs of the southern North Sea. The dunes that built those sandstones were huge, some probably over fifty metres tall. You can see them clearly from where you sit. But dunes are just sand: loose, weak, unconsolidated. Think of a sandcastle on the sea’s edge; it doesn’t take many feeble ripples to plane it flat. But these dunes were not planed flat.
Two hundred and fifty million years later, if you visit quarries where these fossil sand dunes are now exposed in section, you can clearly see how the first bed deposited on top of them by the new sea drapes over those ancient sand hills’ original shape. Moreover, the parts of the dunes closest to the interface with water-laid sediments – where the sand could be expected to have been reworked into beaches as the transgression gathered pace – lack any of the features of beach sands (typical bedding, or the shells that should be found if the beach had existed for any length of time). There are not even any fossil burrows. The inescapable conclusion is that the desert became the bottom of the sea far too quickly for normal shallow water features to be established – or even for the waves to plane the dunes flat.
By the time the great flood was over, the new sea was nowhere more than 250 metres deep. That means that the amount of water needed to fill the entire basin (stretching all the way from Yorkshire to Poland and Russia) would be about 110,000 cubic kilometres. All this would have passed through one narrow northern channel; so the greatest uncertainty in the equation comes in estimating how wide and deep that channel was. If it were ten kilometres wide at its narrowest and allowed a flow twenty metres deep travelling at about three metres per
second (a rather conservative estimate), the water could have rushed in at the rate of about fifty cubic kilometres per day. The whole process of filling the new sea would have taken about six years.
Six years seems long enough, but the rate at which the process was completed is not important when we are confronted by rocks in outcrop. Face to face with the record of events at a single locality, the question is: how fast could any single dune have been covered by water? Even using our conservative estimate of influx rate, it would seem that in what is now Yorkshire, the sea would have risen by some tens of centimetres a day – enough to bury a fifty-metre-high dune in about eight months. One season and the dune sea below you would have become the sea bed, its once sunstruck curves draped in black mud.
The suddenness of the inundation also explains other features characteristic of the dune sandstones underneath the first marine sediment. Dunes have a characteristic internal structure, formed as sand grains are blown over the crests to cascade down the lee slope as the dune migrates downwind. This creates a large-scale form of ‘cross-bedding’, measurable in metres; the fossil dune surfaces forming great rococo festoons and swags.
The odd thing about these particular dunes is how many of them now appear to lack this characteristic bedform, especially at their centres. Here the laminae of sand are often either contorted and chaotic or have vanished completely. For a long time geologists were at a loss to explain this; but the emerging tale of the dunes’ sudden inundation provided an explanation. As the dunes were buried, large pockets of air became trapped at their hearts. Eventually, as the water got deeper, this trapped gas would eventually overcome the strength of the sediment confining it and be released suddenly, disrupting all the original bedding of the sand as it escaped.