SALT: Tropical seas to frozen roads

Driving across the flat Cheshire Plains, you spy mounds of pinkish-grey rock on the distant horizon. Tens of metres tall, they dwarf the industrial buildings around them. And as you approach one, there is the unmistakable tang of salt in the throat and nose, as if you are nearing a stormy sea.

Around 255 million years ago, during the Lopingian Epoch, you would have needed a boat to undertake this journey, not a car. Because at this time, the British Isles, along with much of what is now Europe, was covered by the Zechstein Sea. Because the Britain Isles then lay just north of the Equator, it was warm and dry. Periodic evaporation – or ‘desiccation’ – of this sea caused a layer of rocks called ‘evaporites’ to form. The most common was ‘halite’ – a rock better known as salt.

Halite and other evaporites with exotic names, such as carnallite and sylvite, have been excavated from below the Cheshire Plains for over 150 years. We don’t usually see these rocks at the Earth’s surface because they dissolve in rain, although several hundreds of kilometres of mine workings lie below the Cheshire Plains, where the salt is removed by hand and machine. Beyond their culinary value, these rocks are economically important; they de-ice our roads in winter and some are used to fertilise crops.

Salt will hopefully play a positive role in our economic and climatic future. Its ability to trap fluids and gas make salt caverns ideal sites to store the greenhouse gas CO2, as well as nuclear waste, hydrogen and compressed air, all likely to be important energy sources powering a low-carbon economy.

CHALK: Created from a billion souls

Ice-cold water laps at your toes. You take a step in, gasp, and then look north-eastwards, towards the White Cliffs of Dover. Although shimmering white and ice-like, these rocks tell the story of a very different place, at a much warmer time.

Unlike sandstone and basalt, chalk is a rock that is born, not made. During the Late Cretaceous, 66–100 million years ago, the UK was covered by a warm, clear, almost tropical sea that teemed with life. Billions of microscopic green algae and animals called coccoliths floated in the shallow waters, feasting on sunlight. When they died, their calcium-carbonate-rich skeletal remains sank to the bottom of the ocean, eventually being buried and compacted to form the chalk that so defines the British landscape.

As this chalk slowly subsided beneath the seabed and bacteria got to work, silica-rich ‘flint’ formed. This hard, dark rock was eventually liberated from its soft, white host by the action of time, wind and waves. Thousands of years ago, this same, unyielding rock was being crafted into hunting arrowheads by Palaeolithic humans. What started off as life was eventually used to bring death.

More related content:

• Explore the fascinating wildlife of a Hampshire chalk stream
• Day out: White Cliffs of Dover, Kent
• Best walks in the South Downs National Park

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SLATE: A rock that roofed the world

The landscapes of North Wales and the Lake District share a wild, natural beauty, with rolling mountains of bare rock, river-swept valleys and glistening lakes. But look closer. Some of the most remarkable scenery in the UK has a more ravaged look, defined by sharp cliffs that flank oversized holes, unnaturally blue, box-shaped lakes, and human-made rivers trapped within metal pipes. This seemingly natural scenery bears the indelible scars of us, for these are the markings of slate quarries. Slate has been quarried in these isles for almost 1,000 years. At its peak in the late 19th century, in Wales alone, 280,000 tonnes of roofing slates were produced every year.

To understand the origin of slate, now found in some of our highest landscapes, we dive into the seas of the Palaeozoic Era, 350–500 million years ago. Here, mud accumulated in the quiet waters, eventually being buried, heated and squeezed to form mudstone and eventually, slate. Slate is a metamorphic rock – but only just. Look closely enough and the ghosts of now-extinct sealife remain visible, as do the original, millimetre-thick layers or ‘laminae’, recording the sedimentary snow that dusted the ancient sea floor.

Alamy

LIMESTONE: Creating legendary caves

The narrow steep-sided valley looks out of place among the low rolling hills of north-east Derbyshire. Poking out of the valley side, here at Creswell Crags, is a pale-grey cliff of limestone, dotted with little doorways. You enter one of these rocky entrances, and as cool air strikes your face, you are transported back several tens of thousands of years, during one of the many last Ice Ages that shaped our land, sitting among nomadic people who occasionally called this place home. Rock caves such as this have long provided refuge for humans, sheltering us from the worst of the weather and protecting us from hyenas that used to roam this part of the north Midlands.

Limestone was deposited about 250 million years ago, in the latter part of the Permian Epoch. Deposited by ancient sea creatures in a shallow, warm tropical sea, and subsequently chemically modified by a process called dolomitisation, this is ideal cave-making material. Carbonate rocks, such as limestone or dolomite, make cave-making easier, dissolving naturally on contact with carbonic acid, the product of rainwater dissolving CO2 from the atmosphere. Erode this landscape with rivers cascading from the snouts of ice sheets that extended across much of the UK, and you have the perfect recipe for caves.

The unique properties of the magnesium limestone not only left behind a landscape well-suited for human occupation, but the thin, lime-rich soil that develops as it weathers provides a rich habitat for wildflower meadows. And quarrying of these rocks produces hard-wearing bricks that have built our cathedrals and roads, and enriched our soils. A landscape built by both fire and ice, ultimately protecting us from both, and providing for us, even now.

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