Paths across the Cheshire Peak

Driving west across the edge of the English Peak District is a good way to see how geology shapes landscape. Tracing the routes that cross it – feeling their shapes with a finger on a map or with your body as the car swings round bends – hints at how they are shaped by the landscape beneath, but also the intentions of the people who first made them. Paths across the Cheshire peak were shaped by dramatic changes across both human and geological history.

http://www.geograph.org.uk/photo/1093601

The new Buxton road winds below Shining Tor. © Copyright Jonathan Wakefield and licensed for reuse under this Creative Commons Licence

Roads in Derbyshire’s ‘White Peak’ are shaped by the limestone beneath; they sit in the bottom of ‘dales’ – steep gorges etched into rock – or wind across a bucolic landscape of green fields tessellated by white stone walls. But drive out of Buxton on the Macclesfield (“Cat and Fiddle”1) road and you suddenly enter a wilder world. Within a few feet, the stone walls at the side of the road turn from pale grey to a buff beige.  The landscape is brown and open, empty under a sky that is rarely entirely blue, mantled by peat bog and growing little but heather. You are entering one of the ‘Dark Peak’, one of the wild moors of northern England, the wuthering heights where Heathcliff roamed and Ted Hughes’ hawk roosts.

Further west, at the edge of the moors everything changes again – the Cheshire Plain appears laid out for inspection. On a clear day – or better still night – the view takes the breath away. The homes and lights of 3 million people twinkle and beguile. The depth of detail invites you to study, to pick out Jodrell Bank, flights descending into Manchester airport, Alderley Edge….

The Buxton road climbs up out of Macclesfield. Taken near Toll Bar Avenue.

The Buxton road climbs up out of Macclesfield. Taken near Toll Bar Avenue.

Drivers shouldn’t enjoy the spectacle: this road needs your full attention. It’s popular with motorcyclists for its many bends. Sadly some are total idiots, making the A537 one of Britain’s most dangerous roads. Their attitude to the area is not much different from many other modern travellers – this is a place to enjoy yourself in. Older generations – those who made this and other routes – had other motivations.

 

 

Early trade-routes

Some old routes over the high moors of northern England are know as the Saltways. The ‘wiches’ of Cheshire: Nantwich, Northwich and Middlewich, are towns based on salt. Thick layers sit deep within the Cheshire Basin, formed as a shallow sea was repeatedly evaporated under the Permian desert sun.

Salt has been produced in Cheshire since at least Roman times. An important commodity essential to food preservation (cheese! bacon!) it was transported across the country by salt traders (“salters”) who name attached to their routes. Below is my inference as to the route taken by salters through this area, passing through Saltersford Hall.

Trace of Salters way

Trace of Salters way in green. Macclesfield is the town on the left, Buxton on the right. The brown area in the middle is the moor

Buxton was a Roman town  and a direct line from the salt towns to there passes this way, but there is not good evidence it is that old. These routes are pre-industrial though, used by men and horses walking through the landscape, at the mercy of the elements. A reminder of how perilous this could be – hard to remember when speeding in a warm car – comes from an odd memorial stone on the route, that reads: “Here John Turner was cast away in a heavy snow storm in the night in or about the  year 1755. The print of a womans shoe was found by his side in the snow where he lay dead”

http://www.carlscam.com/rainow/turner.htm

The front of the memorial stone. Source

 The modern age approaches

Even as John Turner grew cold in the snow,  the epoch-making2 Industrial Revolution was hotting up. Using new technology to centralise production in factories only make sense if you can then get your goods to the people who buy them – new forms of transport were an important factor.

The 18th Century – early on in the Industrial Revolution – innovation came in the form of many new roads, called turnpikes. These were independently financed toll roads, sanctioned by Acts of Parliament.

The first Macclesfield-Buxton toll road followed an old route, was ‘engineered by a blind man, John Metcalfe’, and opened in 1759. Initially controversial, it was opposed by some (local coal producers) but championed by the new industrialists.

Old Buxton road in blue, new road in red. Route over Shining Tor in brown.

Old Buxton road in blue, new road in red. Route over Shining Tor in brown.

This road and the salters way are both direct but steep. This is ideal when moving goods with pack horses, but horse-drawn wagons work better with more gradual slopes, even if the route is longer. By the dawn of the 19th Century, new road-building techniques had emerged that cut into the hillside to make wider carriage-ways that avoided steep slopes even over hilly terrain.

New and old Buxton roads cross the far hillside. Cat and Fiddle pub right hand skyline

New and old Buxton roads converge on the far hillside. The Cat and Fiddle pub is on the right hand skyline

In 1808, a new Eddisbury bypass just above Macclesfield was built by the famous engineer Thomas Telford, the “Colossus of Roads”. In 1821 the rest of the Macclesfield-Buxton road was modified with new winding flatter routes and a pub for the weary. The new road is wider than the old and climbs more gradually. To achieve this is has many bends, which attract the loonies in leather on their motorcycles.

A new ancient road

The transition from White to Dark peak, as you go east from Buxton is dramatic to us, but the incoming darkness would have been felt much more keenly by the Carboniferous inhabitants. The sparkling tropical seas where trilobites frolicked in crinoid forests were suddenly snuffed out by the arrival of massive amounts of sand and mud. Rocks made from the remains of life are replaced by those where fossils have to be sought out – traces in sand, crushed shells in rare marine muds or eventually, coal.

The most modern path across the Peak is also the most ancient. Walking across these hills for pleasure is extremely popular and paths easily cut into the soft peat. The most popular routes are now paved with big slabs of the local sandstone – along Shining Tor there are hundreds of them. Covered in ripples and the traces of burrowing bivalves, walking along these makes you feel like you are on a sandy shore 300 million years ago.

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Slabs of sandstone along Shining Tor

 

Ancient ripples

Ancient ripples

"Lockeia" - traces of burrows from bivalves

“Lockeia” – traces of burrows from bivalves

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.

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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.

New Scottish Oil field discovered (470 million years too late)

Scottish oil is topical. Most of Britain’s oil and gas deposits sit under the seabed around Scotland but the revenues are shared with the whole of the United Kingdom.  If Scotland decides to become an independent state (there’s a vote in 2014) then that wealth will be all theirs. So I was very interested to read about a new Scottish oil field that has been discovered. There’s only one reason you’ve not read about this in the papers: all the oil was boiled off 470 million years ago.

easdale slate bgs

Easdale Slate sample. From British Geological Survey, sample P519560

Oil deposits form from dead critters – buried organic matter. Bury this carbonaceous material deep enough (often in black mud) and it heats up and enters the ‘oil window’. These ‘source rocks’ then produce oil which seeps away. In ideal conditions it enters a rock rich in holes (the reservoir) and is prevented from rising further by impermeable rock layers above (the seal).

Most oil deposits are found in more recent rocks, from the last 541 million years where traces of life are everywhere (the Phanerozoic). We know from rare fossils and geochemical evidence that life was abundant before this time, it was just mostly microscopic bacteria or algae. The ‘Cambrian explosion’ is rightly celebrated for the creation of new lifeforms, but its impact is partly due to innovations like hard shells and burrowing in sediment that made ancient life much more visible. It doesn’t necessarily represent a step-change in the raw *volume* of life. Before the Cambrian, there was lots of carbon being ‘fixed’ and sinking into sediment – many oil deposits are found from the next oldest period, the NeoProterozoic (1,000  to 542 million years ago).

Scotland contains sediments of this age: the Dalradian Supergroup. Some clever chaps from the University of Aberdeen thought to look in them for evidence of oil. In their recent paper Timothy Bata and John Parnell focus on rocks from the Argyll Group – the Easdale Slate and the Scarba Conglomerate.

Figure 1 from Bata & Parnell 2013

Figure 1 from Bata & Parnell 2013

The Easdale slate is a dark rock that even today contains up to 6.3% organic carbon by weight -it was a good candidate for a source rock. The Easdale sediments formed in deep water and the Scarba Conglomerate was the equivalent shallow water deposit. As a coarse pebbly sandstone it would have contained many small holes, up to 11% by volume, and so was a good candidate for a reservoir rock. Today it is strikingly dark in colour because it  contains abundant solid hydrocarbon residue – it is a fossil oil reservoir. The residue is found within pore spaces and is associated with pyrite crystals which they interpret as forming from Precambrian bacteria attacking/eating the oil.

These rocks are found across Scotland and Ireland – our authors estimate they could have contained over 6 billion barrels of oil. This find isn’t going to affect the vote for Scottish independence in September though. The Iapetus ocean these sediments were deposited on the edge of is long gone and so is the oil. It wasn’t extracted by cunning trilobites but was destroyed along with the ocean. Around 470 million years ago the sediments were buried and heated to high temperatures – the Easdale source rocks were converted from muds into slates useful in roofing. Only useless degraded hydrocarbons remain, the rest would have been returned to the surface as gas.

Rocks equivalent to the Dalradian might be expected to have similar deposits and these are found from Greenland to North America. Other Precambrian fossil oil reservoirs are there to be found – if you live on lightly metamorphosed Neoproterozoic sediments in eastern North America or in Norway, you might be sitting on the ghost of an oil-field.

Bata T. & Parnell J. (2014). A Neoproterozoic petroleum system in the Dalradian Supergroup, Scottish Caledonides, Journal of the Geological Society, DOI:

The Great Ordovician meteor shower

Between Mars and Jupiter, 470 million years ago, there was a massive collision between two 100km-sized chunks of rock – this solar system’s biggest bang of the last billion years. It created a massive cloud of smaller fragments. Some of these landed on the earth, falling at a rate at least a hundred times greater than at present. These fragments can be found today in sedimentary rocks from that time. More speculatively, this shower from space has been linked to two dramatic events – a set of vast fossil landslides plus a major event in the history of life – a landslide of new fossils known as the Great Ordovician Biodiversification Event.

Grains raining from the sky

Meteorite from Thorsberg Quarry. Image from Lund University

Meteorite from Thorsberg Quarry. Image from Lund University

It started with unsightly green blobs in a Swedish limestone quarry. Discarded by the quarrymen, these odd lumps are fossil meteorites. These are incredibly rare – something most geologists will never see. This may explain why they were only identified correctly in the 1980s (by amateur geologist Mario Tassinari). Since then, researchers – notably Birger Schmitz of Lund University in Sweden – have found over 90 meteorites from this one quarry1.

Recognising this as something remarkable, they started looking for other evidence in other rocks of the same age. Dissolving limestone in acid, they were able to pick out tiny grains of chromite. This mineral forms on the earth, but using chemistry they were able to show that these grains could only have come from space. Such grains have been found in China2 and Russia, as well as Sweden. Tiny tiny meteorites (fabulously called cosmic spherules) have been found in Scotland3 and Argentina.

Reading up on this, I was rather excited to realise that rocks of the same age are found in Ireland. I was writing a post about the use of heavy mineral analysis in these rocks, showing variation in the number of chromite grains! Had I just made an exciting connection? The Irish papers interpret the chromite as coming from an eroded ophiolite – were they actually from space? I did some maths and as it turns out, no they weren’t. Maths can be cruel.

The rocks in Sweden contain so many meteorites for two reasons. As well as forming at a time when huge numbers were falling to earth, they also formed extremely slowly. Known as a condensed sequence, each centimetre thickness represents tens of thousands of years of deposition. Sitting on a flat sea-bed, little or no sand was washed in so only limestone mud, a sort of organic dandruff, settled to the sea-bed. That and fragments of a space collision, scientific manna from heaven. The Irish rocks built up thick layers a thousand times faster as sand, gravel and mud spilled from the hills above. If you processed 1000kg of Irish rock, you’d expect to find only a few grains of extraterrestrial chromite. So the many grains counted in the Irish studies could have contained only a small number of space chromites. The most likely small number being zero.

Look to the heavens

All the best scientific stories reach across different disciplines. Studies of the chemistry of this space dust show it to come from a single source – the meteorites are all of a well-known type called “L-chondrites”.  Using isotopes, Schmitz was able to assess how long his chromite grains had been floating in space, exposed to cosmic rays. The younger the rock layer, the longer the exposure. All of this suggested the middle Ordovician meteorite shower was caused by a single event, fracturing a large body into many pieces. Independently, a 1964 study of L-chondrite meteorites had identified a ‘shock age’ of around 470 million years ago – providing independent evidence for the collision. Using spectral analysis of asteroids (looking very very carefully at their colour), its possible to identify pieces of the original body that remained in stable orbits – the Gefion family of asteroids. L-chondrites even today form about 20% of the meteorites that reach earth.

Hidden impacts?

In the Earth Sciences, things tend to follow a power-law distribution. For example, tiny earthquakes are very common, moderate ones common, large rare, and very large earthquakes are very rare. Smash a huge asteroid into pieces and you might expect the size of the fragments to follow a power-law distribution . On earth we’ve found the uncountable numbers of tiny chromite grains and a lot of small meteorites – it is entirely reasonable to assume that a few crater-forming-size fragments also hit the earth in the Ordovician.

They’ve found a few – the Lockne crater in Sweden and the Osmussaar breccia in Estonia4 are pretty solidly linked to large impacts by L-chondrite bodies in the Ordovician. However craters are remarkably hard to preserve so maybe there aren’t that many more to find. What is needed is traces of the impacts that affected a large area and might be found in sedimentary rocks of this age.

John Parnell of Aberdeen University has suggested5 that the many impacts at this time caused an unusual series of ‘mass wasting’ events on continental margins – essentially a series of massive landslides. These are not small things – the Buttermere formation6 in England’s Lake District is a 1500m thick sequence of sediments that was sheared and folded as they shifted down an ancient sloping sea-floor. There are another 13 similar deposits of middle Ordovician age around the world.

Sample of the Buttermere Formation Olistostrome from Ian Stimpson

Sample of the Buttermere Formation Olistostrome from Ian Stimpson

Not everyone agrees7. Massive landslide deposits are not uncommon – the middle Ordovician was also a time of sea-level fall, something that can cause continental slopes to become unstable. Further, all of the examples formed in tectonically active areas. The Lake District rocks formed on a volcanic arc near a subduction zone. There would have been plenty of large earthquakes to trigger a landslide – there is no need to invoke a nearby meteorite impact to explain it.

Change the history of life?

Could the mid-Ordovician impacts have changed the course of life on earth? In 2008 Birger Schmitz 8 linked them to a dramatic event in the history of life: the Great Ordovician Biodiversification Event (GOBE). Spanning 25 million years, this event saw an unprecedented increase in the number of species of fossil animals. Schmitz and co-workers tracked both fossil abundance and the record of extraterrestrial debris on a bed by bed scale.

Figure 3 from Schmitz et al. (2008). "The results are based on bed-by-bed collections at eight localities. Note the dramatic increase in biodiversity (black line) and high extinction (blue line) and origination (red line) levels following the regional Volkhov–Kunda boundary, that is, the same level where extraterrestrial chromite appears and Os isotopes change"

Figure 3 from Schmitz et al. (2008). “The results are based on bed-by-bed collections at eight localities. Note the dramatic increase in biodiversity (black line) and high extinction (blue line) and origination (red line) levels following the regional Volkhov–Kunda boundary, that is, the same level where extraterrestrial chromite appears and Os isotopes change”

How could the things be linked? They talk of “impact-related environmental perturbations” which feels like one of those CIA euphemisms for murder, meaning as it does “sterilising large areas of the earth”. The key point is, not all of the earth. Once the dust has settled, a habitat empty of inhabitants is a fantastic opportunity for nearby animals to move into. By creating a more varied environment, impacts can actual increase the diversity of species, they argue.

It’s a lovely idea,but one that is far from being proven. Other environmental factors (very high sea-level, lots of islands, changing climate) or biological changes (the GOBE sees planktonic life become important for the first time) are equally or more plausible. In a great recent podcast overview of the GOBE, David Harper (second author on the paper) refers to the meteorite link only after discussing all the other possible causes, and does so with a slightly apologetic tone.

Showing that the GOBE and the remarkable flux of space debris happened at the same time is not enough. What is required is to prove the causal relationship. This is a very hard thing to do. Maybe all of the proposed causes were each partly responsible?

The History of the Earth is history

The Earth Sciences are unusual in being partly a study of past events. Sometimes perhaps we should think more like scholars of human history. Historians studying, say, the origins of the First World War are aware of the importance of multiple causes. The tensions of imperialism, the aggressive German foreign policy, even the inflexibility of railway timetables – are just some of the many proposed ’causes’ of this terrible war. Any educated discussion of this topic would acknowledge that many things were contributory in some way. There is no single ’cause’ for WW1.

Was the extinction of the dinosaurs caused by the eruption of the Deccan Traps or the Chicxulub impact? Yes. Surely both are part of the story in some way9? Thinking a little like historians, the quest to prove one cause is right and the other wrong seems a little foolish.

The events of the middle Ordovician are less dramatic but illustrate the same point. Maybe we will never know if the Buttermere olistrostrome was caused by a meteorite impact, or just a large earthquake and low sea-level. But maybe we will. Historians derive new insights from studying archives of old documents. Our archive is the world itself (and beyond). New rocks, new techniques and novel combinations of the two may one day give more dramatic insights on the Great Ordovician meteor shower.

References

I first heard of the Great Ordovician meteor shower from Ted Nield’s excellent book Incoming, which I recommend to you as it covers all manner of marvellous meteoric matters.

I’ve tried to put web links to copies of the original papers in the footnotes. Formal references to key papers are below.

Schmitz B., Tassinari M. & Peucker-Ehrenbrink B. (2001). A rain of ordinary chondritic meteorites in the early Ordovician, Earth and Planetary Science Letters, 194 (1-2) 1-15. DOI:
Parnell J. (2008). Global mass wasting at continental margins during Ordovician high meteorite influx, Nature Geoscience, 2 (1) 57-61. DOI:
Meinhold G., Arslan A., Lehnert O. & Stampfli G.M. (2011). Global mass wasting during the Middle Ordovician: Meteoritic trigger or plate-tectonic environment?, Gondwana Research, 19 (2) 535-541. DOI:
Schmitz B., Harper D.A.T., Peucker-Ehrenbrink B., Stouge S., Alwmark C., Cronholm A., Bergström S.M., Tassinari M. & Xiaofeng W. (2007). Asteroid breakup linked to the Great Ordovician Biodiversification Event, Nature Geoscience, 1 (1) 49-53. DOI: