Origin of Marine Drive rocks

The rocks on Marine Drive consist of up-ended and folded layers of sandstone and mudstone which were originally laid down on the bed of a deep ocean, thousands of kilometres south of the equator.  How they and similar rocks came to form the present day Isle of Man is a story involving almost half a billion years of the Earth’s geological history.

The original ocean was called Iapetus and lay between two ancient continents, one centred around the south pole called Gondwanaland and the other north of the equator called Laurentia.

As happens on all continents, the upland areas in the interior of Gondwanaland were slowly eroded and rivers carried the eroded material – sand and mud – to the coast.  Some of this material was washed out as sediment into the deep ocean where it collected as flat layers on the sea bed.  From 490 to 470 million years ago, whilst the sea bed slowly subsided, tens of thousands of layers were deposited.  The hard sandstone layers which form much of the cliffs at Marine Drive were deposited from sporadic submarine avalanches but most of the time a gentle rain of fine sediment led to the slow accumulation of layers of mud.  As the layers were buried, the tremendous weight of the overlying layers pressed most of the water out of the sediment until they became rocks.  The thin beds of mudstone which separate the layers of sandstone on Marine Drive were originally around ten times thicker before they lost most of their water.

The Earth’s continents move slowly around the globe by a process called plate tectonics, causing oceans to expand and contract over tens of millions of years.  For example, the present day Atlantic Ocean is widening by several millimetres each year.  The converse to this is where an ocean shrinks to the point that two continents collide (albeit very slowly).  This is happening present day at the juncture between India and Central Asia – the collision zone marked by the Himalaya mountain belt.  The Himalayas themselves are mostly made up of layers of sandstone and mudstone which were originally deposited on the bed of an ocean, just like Iapetus.  The collision between India and Asia caused the layers to be bulldozed together, building a chain of contorted rocks tens of kilometres thick.  The same thing happened to the Iapetus Ocean when Gondwanaland collided with Laurentia around 410 million years ago.  Incredibly, part of the join between the two continents is exposed at Niarbyl, the rocks in the bay having originally been deposited on the seabed close to Laurentia whereas, like most of the Island, the rocks inland are from the Gondwanaland side of the original ocean.

The continental collision formed the Caledonian mountain belt, the remnants of which stretch from North America to Norway and include the Isle of Man, the Lake District and the Highlands of Scotland.  Originally there were several kilometres of rock overlying the Isle of Man but these have been eroded by water, wind and ice.  The effect of the collision was not only to fold and up-end the layers of rock but also caused the roots of the mountain to melt due to very high temperatures at depth.  These molten rocks rose upwards as gigantic balloons of magma under intense pressure, an example being the Foxdale granite, the granite representing magma which cooled and crystallized when it reached shallower depths.

The newly joined continents gradually drifted north until the Isle of Man reached its present position.  On its way, the Peel Sandstone was deposited in desert-like conditions and the Castletown Limestone in a shallow, tropical sea.  Around 60 million years ago, the North American-European continental region rifted apart to form the Atlantic Ocean.  This tectonic upheaval caused the whole Irish Sea region to rise another 1-2 kilometers as the weight of America was unshackled.  This uplift caused rivers to downcut again, resulting in the Manx glens.  Also the tall cliffs along the coast, including Marine Drive, partly owe their origin to this upheaval.

The penultimate geological event affecting the Isle of Man was the Ice Age.  The ice melted around 12,000 years ago leaving behind a landscape that had been scoured by glaciers, giving it a distinctive rounded appearance.  We have recently entered the Anthropocene period, the name given to the time when the natural environment is being significantly affected by the activity of humans.  This means that the road along the Marine Drive is now officially part of the geological history of the Isle of Man!