Traces of glacial ice and water

There’s an immediacy to the study of the Quaternary (the last few million years) that is rather seductive. Most geology is (after John McPhee) studying ‘the former world’ but the Quaternary is close enough in time that it is still this world, capped by ice and full of familiar animals and human beings. We can study this period of time in tremendous detail using things – piles of sand, the pattern of the landscape, peat bogs – that are unlikely to be preserved in the geological record.

An outcrop of Irish gabbro tells us about conditions deep within the earth, but the mountain range, even the continent it formed in are all gone. The smooth shape of the outcrop and its covering of fine scratches were caused by the scraping of stones in ice, part of a massive icesheet that stretched across the British Isles. The ice is gone but it flowed over this hill, down that valley. On a chilly day it can feel like it only just left.

Stone moved by ice

One outcome of the great ferment of ideas in 19th Century Britain was the recognition that much of the northern British Isles were once covered by of thick sheet ice. One of the earliest recognised forms of evidence for these vanished ice sheets is found in the form of glacial erratics. These are pieces of rock, sometimes very large, dumped by the ice. The most useful sort come from a distinctive rock type, a granite intrusion perhaps, that allows you to know precisely where the erratic came from and so infer which way the ice was flowing. On the Yorkshire coast in England there are erratics from Norway1, showing that the ice flowed across what is now the North Sea.

Freshly dug glacial drift from Cheshire.

Freshly dug glacial drift from Cheshire.

Volumetrically the biggest record of glaciation is glacial drift. This is sediment that was moved and ground-up by the ice. It is a very jumbled, poorly-sorted sediment, with big blocks mixed up with sand and silt. If you find a sediment like this, you know there has been glaciation. This applies to ancient sediments just as much as recent ones.

Studying drift, people realised that things were quite complicated. A single place might have multiple layers of glacial drift separated by more normal sediments. They realised that term ‘Ice Age’ is a simplification; this was phenomena that pulsed. Outside of the Polar regions, the ice caps came and went many times, dancing in time with the stately precession of the earth’s axis.

Isoclinal folding in glacial sand and clay. Photo from 1921 courtesy of British Geological Survey. P249721

Isoclinal folding in glacial sand and clay. Photo from 1921 courtesy of British Geological Survey. P249721

Sometimes, soft drift gets pushed around by advancing ice. Sometimes this results in beautiful folds, other times it puts sediments containing marine shells deep inland. 2. For this reason, the presence of drift is fairly uninformative. To make firmer conclusions about the most recent advance of Ice, we must turn to more subtle features.

Fainter traces

Glacial sediments aren’t laid down in thin even layers, but in various ways, both elegant and ugly. Valley glaciers often have moraines: piles of sediment at the end or sides that fell out of the melting ice. The same principal applies to Ice Caps, such as covered most of Northern Europe and North America. Successive belts or ridges of moraines can record the retreat of an ice sheet.

Drumlins are piles of glacial sediment that have been moulded by ice flow. They are very beautiful features, with an aerodynamic shape. They can look like the back of a huge whale, somehow rising out of the ground. Often found grouped together, their shape indicates the direction in which the ice was moving when last it was flowing.

A pod of Drumlins swimming in Clew Bay, Ireland. Photo from chrispd1975 on Flickr under CC.

A pod of Drumlins swimming in Clew Bay, Ireland. Photo from chrispd1975 on Flickr under CC.

Glacial striations and polishing are common features found on land that was once under the ice. Stones in the ice slowly scratched their way across bed-rock. Asymmetric features known as roche moutonnée tell us the direction of ice flow.

A flock of Scottish roche moutonee (ice flowing to the right). Image from British Geological SurveyP008317

A flock of Scottish roche moutonee (ice flowed to the right). Image from British Geological Survey P008317

A common experience when walking one of the bigger mountains in Britain is to start in a valley filled with glacial till, perhaps with some moraine visible. Next a climb up a ridge shows lots of polished rock. Finally, the summit pyramid is covered in a great thickness of loose stone3. Between this summit block field and the scraped stone below is the trim line that captures the top of glacial erosion. Map out trim lines on multiple mountains and it tells you something about the vertical extent of the ice4.

Summit block field of Glyder Fawr in Wales. Image courtesy of British Geological Survey. P222636

Summit block field of Glyder Fawr in Wales. Image courtesy of British Geological Survey. P222636

When ice melts, it turns into water. In my gin and tonic this is fine, but when the melting ice is 100s of metres thick, it will have a big impact. Around my home town of Macclesfield in England there are glacial lake deposits. They are sitting above the edge of the Cheshire Plain – there’s no way you could have a lake there today. The only way to explain this vanished body of still water is: it was dammed by the ice.

Other evidence of water flowing in odd ways if found in glacial meltwater channels. These look like small stream beds, but they have no stream today. Sometimes they flow along slopes or uphill for a time -evidence that when the water was flowing, the ice was still around.

If you were building a dam to make a huge lake and you proposed making it out of ice, you wouldn’t get far as an engineer, because at some point the dam will fail and all the water will come flooding out. This happened with melting ice in several places. The huge scoured landscape of the channeled scablands in Washington State, USA, are the biggest example, but my favourite is the Jutulhogget or ‘Giant’s Cut’ in Norway.


Jutulhogget  Image from Wikimedia Commons.

Of limited scientific use, but rather beautiful, iceberg keel marks are more evidence for glacial lakes.

Glacial keel-marks from Canada. Google Earth image.

Aerial view of glacial keel-marks from near Manitoba, Canada. Square lines are roads: see here for more details

 To the science

Knowing about these features really enhances your view of the world – it gives you a way to read landscapes and discover a world of ice so close in time we can almost touch it.  But the best thing about these features is that they tell us about the now-vanished ice. Modern researchers have mapped them to track the ice’s ebb and flow. They combine these maps with computer modelling, insights from active ice sheets and techniques for dating so advanced that they seem almost magical. Their goal is to predict the future. In the face of a changing climate, an ice-cap died in Britain 15,000 years ago. Understanding this process better may help us predict that fate of earth’s remaining ice caps. I’ll write more about this next….

Categories: England, Glacial, landscape, Scotland

Andalucia: a history of stuff

Andalucia is a province in Spain, at the far south west of Europe. Its long and varied human history has seen it linked to the middle East, north Africa and the Americas. The creation of these links brought new foods, metals, diseases: new stuff into Andalucia. Sometimes the impact of arrival created ripples that reached out far across the world.

Geological history

Some 200 million years ago Andalucia was within a massive continent called Pangea, close to both Africa and North America. Slowly Pangea broke into pieces, a new ocean basin – the Atlantic – filling the gap as the Americas drifted away. Some 50 million years ago, Africa was pushed north in Europe creating a long mountain range. Andalucia was part of this. Mountains form when the crust is thickened, pushing rock into the sky. A similar process at the base of the lithosphere 1 also forms a thick balancing ‘root’. Under Andalucia, this thick root ‘fell off’2 and sank into the hot convecting mantle beneath.

Mountain belts are surprisingly fragile. The sudden removal of the heavy root caused the over-thickened crust to collapse, flowing sideways and bringing deeply buried rocks up to the surface3. Fragments of mantle called peridotite, not often seen at the surface, form brown mountains around the town of Ronda. The collapse of the mountain belt went so far that its centre is now the very western part of the Mediterranean, the Alboran Sea. The mountain became a great hole in the ground.

Most of Andalucia is made up of sedimentary or metamorphic rocks folded and twisted by these dramatic changes. An exception is the basin of the Guadalquivir river. Here the weight of the collapsing rocks pushed down the rocks to the north, making a depression that has filled with recent sediments – a feature called a foreland basin. The edge of this flat basin makes a clear line that is easily visible from satellite views of Southern Spain.

From Wikimedia

Andalucia from Space. Image from Wikimedia 


People, at first hominins such as Neanderthals, have lived in Spain for over a million years. The little we know of prehistoric humans comes from their use of materials. First their gradually more sophisticated use of stone tools, then from about 5000 years ago the smelting of metals: first copper, then bronze (copper plus tin) and then iron. Mines in Andalucia have been been involved since the start, notably the Rio Tinto area. Here a Carboniferous massive sulphide deposit has yielded silver, gold and copper and spawned a global mining company.

Image of Rio Tinto mines, Andalucia. Image from David Domingo on Flickr under CC

Rio Tinto mines, Andalucia. Image from David Domingo on Flickr under CC

Around 3000 years ago (1100BC) the Phoenicians reached Andalucia, founding the town of Cadiz. A culture that reached across the Mediterranean they were also involved in trade with the British Isles. Tin from Cornwall in England was smelted with Spanish copper and the resulting bronze traded on. The olive tree reached Spain at this time, brought from the eastern Mediterranean.

A mere thousand years later, the Romans took control – their province of Hispania Baetica covers much of modern-day Andalucia. They introduced deep mining to the Rio Tinto, using the characteristically Roman combination of slaves and very big wheels to pump water up from the depths. Andalucia was a renowned source of many products for the wider Roman Empire, including silver, olives, emperors, philosophers, dancing girls, and garum, a sauce formed from fermented fish guts rich in umami flavours.

Yet Moor invaders

After the slow collapse of the Roman Empire, the next major influx of change in Andalucia was the Moorish invasions. The Islamic Moorish army4 conquered most of Spain between 711 and 718 AD, in time creating a kingdom of Al-Andalus with its capital in Cordoba. Once more Andalucia was part of a multi-national empire with good trade links. Valuable crops from further east, such as figs, citrus and pomegranate were introduced for the first time, as were sophisticated irrigation systems, some of which are still in use. By the 10th century, Cordoba was the most civilised city in Europe, it’s Grand Mosque was one of the wonders of the Muslim world.

Cordoba Grand Mosque

The site of the Grand Mosque was originally a Christian cathedral (before that, a Roman temple). Built over 200 years and funded in part by mining proceeds, the Mosque was built with local stone and brick, but also recycled Roman stone columns. Some of these were found locally, but others were brought in from much further afield, from the rest of Spain and perhaps wider.

The Columbian exchange

Eventually Moorish Spain came to an end as Christian rulers conquered the Muslim lands. The final Moorish kingdom of Granada fell to Ferdinand and Isabella in 1492, the same year that they sponsored Christopher Columbus to mount an exhibition across the Atlantic. As  the Spanish kingdoms turned into a global Spanish empire, lots of incredible things started flowing back into Andalucia. First Seville and later Cadiz were the main ports for the Atlantic trade. Gold and silver beyond imagining passed through Andalucia – enough to create a century of inflation across Europe. Some of this stuff ended up in the the Grand Mosque in Cordoba, which is now a cathedral again. Sitting within the vast pillared area of the mosque is a Christian church full of beautiful things made of American gold and silver. The choir stalls are made of American mahogany – lots of plant material crossed the Atlantic too.

Ingot of South American silver as brought over by Spanish treasure ships. Cadiz Museum.

Ingot of South American silver as brought over by Spanish treasure ships. Cadiz Museum.

A popular Spanish dish is called ‘patatas bravas’ and consists of potato, tomato and chili – all foodstuffs that spread across the world following the ‘discovery’ of the Americas. Andalucia was the first stop for many of these vegetable treasures. Botanical gardens turned seeds into plants, to be studied and propagated. A fine building in Seville is the old tobacco factory, dedicated to processing another new crop. The setting of Bizet’s opera Carmen, its walls are mottled with yellow and brown, like a smoker’s fingers. More immediately bad for the health, syphilis was first recognised in Europe in 1494, most likely brought back by Columbus’ sailors.

The British dimension

As an Englishman who likes history, I often visit other countries in an apologetic mood. A dimly remembered story about Francis Drake daringly ‘singeing the king of Spain’s beard’  is rather less jolly when you are sitting in Cadiz, the town that was attacked. Drake was engaged in warfare on behalf of his Queen, but also behaving like a pirate, raiding Spanish treasure ships. Still, no one seems to mind any more; it was in 1587 after all.

The British drink everything and anything. Not content with home-grown beer, gin and whisky we also crave grape-based booze. When Francis Drake returned from attacking Cadiz, he brought 2,900 barrels of sherry, a type of wine made only near Jerez in Andalucia. This went down rather well – we’ve been drinking it ever since, even getting involved in its manufacture.

Tonic water is sweet fizzy water flavoured with quinine, best taken with gin. Quinine, an extract from a South American tree was for centuries the only effective way of countering the effects of malaria. First popularised by Spaniards returning from Peru it was introduced to gin by the British in India. In the early 18th century the Royal Navy had a number of bases on Spanish soil, including in Andalucia, and passed the habit on. It remains popular in Spain to this day.

Image from Amanda Slater on Flickr under cc

Image from Amanda Slater on Flickr under cc

A final anglo-Spanish connection is marmalade. Seville is full of orange trees, of a particular kind, bitter and rich in pectin. They are pretty inedible raw, but for some reason 5A Scottish tradition and industry grew up of preserving them as jam, with pieces of the skin floating in jars of pungent and yielding orange delight. Paddington Bear, James Bond and Alice in Wonderland all eat marmalade, along with many real people, such as me. The bitterness gives it a very grown-up feel, with some of the grimy delight of cigarettes and whisky, but in a healthy breakfast-friendly form.

The future

What of the future? Recent research suggests that soon6The oceanic crust beneath the Atlantic will start to plunge down under Spain into the earth’s mantle. The collapse of the mountain belt I mentioned at the beginning left a tear in the crust and this may grow and extend into a full-blown subduction zone. This will bringing volcanoes to fertilise the soil with ash and earthquakes to shake the buildings (if any remain). As slowly as finger-nails grown the Atlantic will vanish and Spain and the Americas will be reunited once again.

Categories: History, mountains, subduction, tectonics

Deep time and dead things in the wall

Here in Cadiz, I’m surrounded by death and I don’t mind at all.

It’s the geologist in me thinking this – in every other way this is a lovely life-affirming holiday. It’s just that every surface seems to be filled with the remains of long-dead animals.

The floor and one wall of the hotel room in which I’m sitting is covered in a fine white limestone packed with the remains of fossils – mostly bivalves. There are thick finely layered oyster shells and finely ribbed shells that remind me of scallops. The stone has mostly been cut perpendicular to bedding so the shells are seen in cross-section. These elegant fine grey lines are within a lighter matrix that – on closer inspection – is made up of smaller pieces of something-that-once-was-alive-and-now-is-not.


This is of course a wonderful thing. To be surrounded by traces of an ancient sea-bed, as productive and vibrant as any modern tropical sea and to live in a time that fully understands that significant of these little grey squiggles is indeed a privilege. But they are still dead.

Leaving the room doesn’t help. The city of Cadiz seems largely built from the local rock. This is basically a pile of shells bound together with ground up shell and maybe a little brown sand.

20140126_160603Cadiz is great for seafood, there were men with buckets of sea urchins and big knives wandering around earlier. Imagine a seafood restaurant kitchen at the end of the evening: there’s a pile of shells somewhere – big chunky oyster shells, purple fragments of bust-open sea urchins, maybe a few spiny sea snails. If you laid them out in a layer they’d cover a few tables maybe, not more. If an eccentric restaurant owner made a pile in his garden, after a year it would be pretty big (and very smelly). Archaeologists call similar features shell middens and given thousands of years, greedy people can make piles that are kilometers long and meters thick and wide. The layer of stone that old buildings in Cadiz are made from covers a much wider area – it’s not just a strip along the coast – so it must be thousands of times the volume.

This is where deep time comes in. My simple thought experiment suggests that the stone that Cadiz is made of took millions of years to form. Similar thinking by 19th Century geologists revealed the unsettling scale of geological time and set the scene for Charles Darwin to formulate his theories. Over these timescales tiny steps can make enormous changes, just as piles of small animals can be used to build a city.

Deep time can be unsettling: what will the remorseless grinding of the years do to us and our achievements? Will everything and everybody you know be reduced to a few odd layers and a puzzling mass extinction? Maybe. Nobody knows. But my life is enriched by knowing my place within the wider framework just as my walk to the restaurant this evening will be enlivened by spotting the oyster shells in the wall.

Categories: sediments

Winds of change

There’s a fabulous new site that shows wind patterns – it gives you a whole new perspective on the globe. One of the most striking things is the regular patterns across the oceans. Until quite recently long-distance travel was dependent on sailing boats, at the mercy of the wind and regular patterns of wind were needed to support regular sailing routes. The goods and people moved by these winds in turn drove dramatic historical changes that still resonate. Let’s look at a few, illustrated by the Earth Wind Map.

Turn of the sea

In the early 15th Century, Portuguese sailors began exploring the Atlantic ocean. Initial footholds were made on groups of volcanic islands: the Azores and the Canaries. Regular travel between these islands and Portugal taught sailors an odd trick: the best route back home was not a straight line but instead a big loop. This ‘volta do mar’ or ‘turn of the sea’ relied on working with the prevailing winds and ocean currents.

Atlantic winds (green), currents (blue) and approximate Portuguese sailing routes (red). Image from Wikipedia.

Atlantic winds (green), currents (blue) and approximate Portuguese sailing routes (red). Image from Wikipedia.

Here’s how the Earth Wind map looked for this area recently: the same pattern can be seen even in this snapshot of actual conditions.

Image captured from Earth Wind Map. With permission.

Image captured from Earth Wind Map. With permission.

Later sailors showed this pattern to be of global significance. It stretches across the entire Atlantic – Columbus used it on his return from the Americas. Eventually Spanish sailors correctly guessed that it applies to the Pacific, allowing the Spanish empire to connect Central America and the Philippines.

These patterns are caused by global circulation of the atmosphere. Warm air rises at the Equator and flows up and towards the poles. At around 30° latitude (north or south) it descends again into a region of high pressure called the Horse latitudes. Further north a different circulation cell is found, forever whirling in a different direction. These flows of air, on a spinning globe, create patterns of prevailing winds: ‘trade winds’. These patterns are fundamental features of the earth and have been recognised in ancient climates.


The Portuguese weren’t the first people to sail long distances, of course. Trade across the Indian Ocean between Africa, the Middle East and India was established long before; it supplied the Romans with spices as well as lions and tigers for their circuses.

The northern Indian, like the Atlantic at the same latitude, has trade winds that move towards the south west. This is convenient if you want to speed your cargo of hungry tigers from India over into the Red Sea and via Egypt to Rome. But how do you get a cargo of gold coins to India to buy the animals in the first place? Unlike the Atlantic, the horse latitudes are on-land: there is no turn of the sea to get you east.

Indian Ocean2

The winter pattern of winds, away from India

In the Indian Ocean something remarkable happens. The billion year old trade winds are switched, flipped completely round every year. From May until October winds blow from the south and west, towards India. This pattern allows repeat journeys and supports trade. In later centuries these winds sculpted a maritime ‘Dhow culture’ that stretched from the East African coast via Arabia into India.

What has the power to overturn a global pattern of wind? The Himalayas and the Tibetan Plateau do. This massive area of high altitude land was formed and is kept aloft by the ongoing collision between Indian and Asia. During the summer, heating of the high land lowers air pressure, drawing in air from the ocean. This reverses the pattern of winds and brings massive rainfall to India, feeding crops that feed millions.

Sugar, gold and blood

European exploration of the Atlantic of course led to the ‘discovery’ of the Americas, a new land full of Silver and Gold and indigenous civilisations collapsing under the onslaught of vicious diseases to which they had no immunity.

For many, the question was: how to make money from this new land? For the Spanish in central and south America, silver and gold was the obvious answer.  The Portuguese in Brazil and the English and others in North America and the Caribbean turned to farming, preferably of addictive substances.

Sugar, cotton and tobacco could all be grown in the Americas and shipped west to newly addicted populations in Europe. But who was going to grow the stuff? Shamefully, the answer was slaves from Africa, sold in exchange for rum or textiles made in Europe (from ingredients grown in the Americas).  Part of the reason this system worked was to do with the winds. The ‘turn of the sea’ was scaled up into the ‘triangular trade’ that took ships from Europe, to Africa, to America and back again.

triangular trade

In time Europeans realised the full horror of this system. The world’s first consumer boycott was of sugar, first organised in Britain in 1792. They realised that not a cask of this slave-grown sugar came into Europe “to which blood is not sticking”.


What the British mostly did with sugar was put it in their tea. Until the mid-19th Century, drinking unboiled water in a British city was a good way to die young. The traditional solution to this was to drink beer or gin – both sterile, if not completely healthy. In time, powered by growing commercial power in Asia, tea replaced booze as the British drink of choice (at least during working hours). This thirst drove the last great flowering of commercial sailing ships: the tea clippers.

Tea is a seasonal crop and the best tea is fresh tea. For these reasons fabulous beautiful ships were built to speed it across the world. The great tea race of 1866 saw four sailing ships race from China to Britain packed with tea. The first ship to arrive would command the best prices. The route they followed was the fastest possible, drawing on hundreds of years of knowledge of the trade winds. Let’s trace their winding route on a windy globe.

From China, down the South China Sea through the Sunda Straight

From China, down the South China Sea through the Sunda Straight

Across the Indian Ocean and round the Cape

Across the Indian Ocean and round the Cape

Back to Blighty, past St Helena and the Azores

Back to Blighty, past St Helena and the Azores

Categories: History, not geology