Long, long ago and far, far away an embryo began to form beneath the waters of the great Iapetus Ocean. This period in the Earth’s history has been named the Ordovician, and it is said that it began 500 million years ago and that it lasted for 50 million years. The Iapetus Ocean, in those far-off times, lay near to the South Pole. On one side of it lay the continents of Avalonia and Baltica, while far away to the north-west lay that of Laurentia, encompassing lands that would one day form parts of North America, Greenland and Scotland. Although Iapetus was still at that time a mighty ocean, the slow process of its destruction had already begun, for the great continents of Laurentia, Baltica and Avalonia were advancing inexorably towards one another, trampling Iapetus underfoot as they went, forcing it down into the depths of the Earth’s mantle. The advance of the continents, though relentless, was mercifully slow, just one or two centimetres a year. In the early years of the Ordovician Period it presented little threat to the development of the embryo, which lay just off the north-west coast of Avalonia, where the waters of the Iapetus Ocean covered the lowest slopes of the continent, forming a broad sea. |
These waters were home to many strange and some more familiar creatures. Shells like little clams clung to rocks on the sea-bed by a single stubby leg, twisting and turning in the current: these were the brachiopods, their jaw-like shells opening and shutting as they drew in sea-water, from which they filtered tiny morsels of food. Creeping among them like miniature, submarine armour-plated tanks were the trilobites, while above them, drifting through the water, were the graceful graptolites, colonies of minuscule creatures, each one housed in a tiny cup from which its delicate tentacles emerged to feed on the plankton.
The water itself was dark and murky, for the rivers which flowed into the ocean carried debris washed from the lifeless, barren lands they crossed, the ancient, eroding rocks of Avalonia. As the rivers entered the vastness of the ocean, their current slackened and they began to drop their load. Slowly each grain of sand, each particle of silt and clay that they had been carrying drifted down to the ocean floor.
The water itself was dark and murky, for the rivers which flowed into the ocean carried debris washed from the lifeless, barren lands they crossed, the ancient, eroding rocks of Avalonia. As the rivers entered the vastness of the ocean, their current slackened and they began to drop their load. Slowly each grain of sand, each particle of silt and clay that they had been carrying drifted down to the ocean floor.
And thus it was that the embryo which lay beneath the waters of the Iapetus Ocean, the Lake District in the earliest stages of its life, began to grow. Day after day, year after year, century after century, millennium after millennium, on and on for 30 million years, the sediment brought in by the rivers slowly accumulated, softly burying millions of generations of sea creatures, millions of brachiopod shells and graptolite casings, and the outgrown and discarded armour of the trilobites. From time to time, huge piles of debris on the gently sloping sea-floor collapsed under their own weight, and swirling underwater avalanches travelled for hundreds of miles, spreading thick swathes of sediment in just a few hours. Under the weight of the accumulating sediment, the slow process of petrification began. Pressed ever more tightly together, the fine particles of mud and silt settled into compact layers as the water was gradually squeezed out. The coarser-grained sands were more resistant to the pressure and water continued to trickle between the grains, dissolving minerals as it went. Eventually, as the weight above increased, these minerals crystallized out, cementing the sand grains together into a solid mass, like damp sugar. |
Thus, infinitesimally slowly, the soft sediments became solid rock: great thicknesses of mudstone, siltstone and sandstone. These rocks, formed beneath the waters of the Iapetus Ocean in the far-off days of the Ordovician Period, are the Skiddaw Slates, the rocks of northern Lakeland, of Grasmoor and Grisedale Pike, of Hindscarth and Blencathra, of Whinlatter, Dodd, and of Skiddaw itself.
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These rocks have many ancient and beautiful stories to tell. Yet their tales are long, full of intricate detail and ambiguities, and permit of many different interpretations.
They tell of the rivers which reached their journey’s end in the great Iapetus Ocean 500 million years ago. As they entered the sea, their energy waned and they began to drop the sediment they had been carrying. First they dropped the coarse, heavy sand, while the finer silt and mud were carried further out to sea. Close study of these rocks has revealed that the rivers approached this part of the ocean from the south-east, although the course they took and the shoreline they crossed shifted and changed many times during the long, slow growth of the Skiddaw Slates. Laminated rocks, in alternating layers of light and dark, coarse and fine, tell perhaps of ancient cycles of spring flood-waters choked with debris, sweeping down from the mountains to the sea, followed by summer droughts, when the river waters dwindled to a mere trickle with barely the strength to carry the tiniest particles of clay. The graceful curves of slump folds record where wet sediment collapsed and slipped down gentle submarine slopes. Turbidites (coarse-grained at the bottom, fine at the top) are the fall-out from the more dramatic underwater avalanches which threatened the lives of the brachiopods and the trilobites, caught up in their swirling currents and smothered in thick swathes of sediment as the avalanche spent its force and died away. Indeed, it is the rocks that tell us of the presence of these creatures in the Iapetus Ocean, for the ocean-floor was their graveyard, where they were slowly buried in sediment and turned to stone. Yet fossils are rare and difficult to find in the Skiddaw Slates, as few survived the crumpling, crushing and intense heat which these rocks endured when the ancient continents of Avalonia, Baltica and Laurentia finally collided. |
For 30 million years the Skiddaw Slates grew in thickness, layer upon layer, until some 3 kilometres of solid rock lay beneath the waters of the Iapetus Ocean. We cannot be sure of the precise moment when the peaceful growth of this giant embryo was first disturbed, but in the northern Lake District there are rocks which tell of a transition to more turbulent times, heralding the approach of a period of great violence which would produce a new family of rocks, fundamentally different in nature to the Skiddaw Slates.
Remember that throughout the Ordovician Period, the three continents of Laurentia, Avalonia and Baltica had been creeping ever closer together. The Iapetus Ocean, which lay between them, was caught in a battle zone. The ocean floor was old and cold, thin and weak, unable to resist the younger and more buoyant continents on its flanks, and it was gradually being destroyed. Huge chasms opened up on either side of the ocean and along their length the ocean floor was slowly pushed down into the Earth’s mantle by the advancing continents. The young rocks of the Skiddaw Slates only narrowly escaped destruction themselves, for they lay not on the deep ocean floor but on the flooded shores of Avalonia, just out of reach of the all-consuming jaws of the deep ocean trench. Now strange as it may seem, the destruction of an ocean takes place so slowly as to escape the notice of all but the most well-informed inhabitants of the Earth. Nevertheless, within the lifespan of a planet this is a dramatic and violent event which has far-reaching and devastating consequences. 470 million years ago the Skiddaw Slates were enduring colossal stresses and strains as the Iapetus Ocean floor was forced down into the mantle beneath them. Tensions would build for hundreds, if not thousands of years, until finally the rocks snapped, sending shock-waves shuddering up through the Skiddaw Slates. The younger layers near the surface, that had not yet fully hardened into rock, were left shattered and broken; the loose sediment above them was thrown up into the water in great swirling clouds; underwater avalanches of semi-petrified rock and mud slithered and tumbled down the continental slope, coming to rest in a chaotic jumble of rubble, over which a shroud of mud slowly settled. |
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Deep within the Earth a great conflagration arose, sufficient to melt the very rocks themselves. Hot, liquid magma worked its way up along cracks and fissures, melting the rocks as it went, to collect in a huge chamber which reached up into the Skiddaw Slates. More and more magma found its way in and the pressure in the chamber mounted until the Skiddaw Slates could contain it no longer and were rent apart. Hot magma gushed out into the coastal waters of the Iapetus Ocean, spreading over the sea-bed like hot treacle until, quenched by the ocean waters, it gradually slowed to a halt. Within a matter of days it had cooled once again to solid rock: a lava flow a few metres thick, formed in a few violent hours, a far cry indeed from the slow, gentle, painstaking, million-year accumulation of the Skiddaw Slates. |
Many more eruptions were to follow. Most were like the first, great outpourings of hot magma, but some were violent explosions of hot ash and rock debris. In the thousands of years which separated the earliest eruptions, a steady rain of mud and silt continued to drift down through the ocean waters, quietly burying successive layers of lava and ash in a mantle of soft sediment. Thus we find, on the northern fringes of the Lake District, that the upper layers of the Skiddaw Slates give way to layers of lava and volcanic ash, interspersed with thin layers of mudstone. The volcano quickly grew in height to form an island and so the mudstones disappear from the sequence, leaving only layers of lava and ash. These rocks are known today as the Eycott Volcanic Group, and are found in an arc which stretches from Eycott Hill, passing behind Carrock Fell and Skiddaw, to Binsey. The magma chamber which fed the Eycott Volcano is believed to lie still beneath Carrock Fell, though it has long since lost its heat and its power, and become cold, hard rock. On the fellside, great sheets of gabbro reach the surface, the exposed remains of conduits of lava which forced its way between the layers of the Skiddaw Slates, driven by the mounting pressure in the seething cauldron far below. Trapped within the rocks as the eruptions subsided, these lavas cooled gradually, so that small crystals had time to form and grow. |
The Eycott Volcano eventually spent its power and became silent, but Avalonia, Baltica and Laurentia continued their unremitting march towards one another. Too buoyant to be dragged down into the depths of the Earth in the wake of the Iapetus Ocean, these three great continents were destined to meet in a colossal
head-on collision. Meanwhile, caught in the vice between Laurentia to the north-west and Avalonia to the south-east, the young rocks of the Skiddaw Slates were slowly being pushed up out of the protective waters of the dwindling Iapetus Ocean. Now they found themselves at the mercy of the elements, of the destructive powers of wind, rain and ice. Streams and rivers began to carve valleys in the newly exposed land surface, and the Skiddaw Slates, built from the debris of an ancient eroding continent, themselves began to be worn away. |
Little is known about this first period of erosion to which the Lake District was exposed in its infancy, for it was followed by events of such devastating proportions as to all but completely mask its effects. Indeed there are those who believe that the Lake District still lay beneath the ocean when the next stage in its development began. Whatever the truth of the matter, there is no doubt as to what happened next: 460 million years ago the full force of the inter-continental battle was unleashed in the first violent eruption of the
Borrowdale Volcano.
Borrowdale Volcano.