Through The Sandglass

Mont-Saint-Michel: a massive sedimentology experiment

  The bay of Mont-Saint-Michel, in the coastal corner of France between Normandy and Brittany, boasts the fourth-largest tidal range in the world – up to 14 meters (46 feet) and the sedimentary drama to go along with it. It is an incredibly complex system. The tides carry sand and silt into the bay and leave much of it behind, augmented by the sedimentary loads brought down to the bay by the three rivers that drain into it. Roughly 700,000 cubic meters of sediment are added to the bay every year – it is, quite naturally, silting up (or, more attractively in French, experiencing ensablement – sanding up). As a result, the iconic World Heritage Site of the monastery towering above its isolated rock is increasingly losing its magnificent isolation and becoming united with the mainland. But this natural process has been greatly exaggerated by human meddling over the centuries and the rhythms of the bay profoundly changed and subdued. The natural processes operate on a scale far beyond that of any human endeavour, but it is possible to mitigate the effects of our meddling and restore the bay to at least close to its unhindered condition and allow natural ensablement to take its course. And that is exactly what is going on today in the bay, after years of preparatory work and research by a syndicate of authorities and institutions – a grand experiment so that Mont-Saint-Michel “ recovers pride of place as the central feature of the Bay, amid a seascape of sands constantly being reshaped by tidal and river water.”

Mont-Saint-Michel began as an isolated rock remaining emergent from the bay as rising sea levels inundated the land following the end of the last ice age; it was periodically connected to the mainland by a natural spit of sand, a tombolo. But then, in 708 so the story goes, St. Michael approached the bishop of nearby Avranches, Aubert (later also a saint) and demanded that a monastery be built on the rock. Foolishly, Aubert ignored this request, finally setting about the task only after the irritated angel had burned a hole in the bishop’s head with his finger (now there’s a warning about procrastination and disobeying orders). The monastery and then abbey grew up over the centuries and then declined; after the French revolution, it became a prison that welcomed, among many anti-republicans, Victor Hugo – he described the tide as coming into the bay “with the speed of a galloping horse.” Along with this tide came the human one – pilgrims both religious and touristic. Today, it is the most-visited site in France outside Paris – three million people visit each year. It was teeming even in mid-September when I arrived, the massive parking lot and causeway (built where the tombolo used to be) filled with cars, camper-vans and buses. The causeway completely cuts off the natural currents and sediment movement of the inner bay, but it is by no means the only manmade disruption. In the 1850s, the channels of the Rivers Sée and Sélune were eroding the agricultural area south of where they entered the bay, and a dike (“digue”) was constructed to force their flow further north; over the following decade, the channel of the River Couesnon was completely controlled and “canalised” all the way out beyond Mont-Saint-Michel itself (the artificial banks for much of this were eroded or overwhelmed by sand by the 1960s). Critically, in 1969, a dam was built across the Cuesnon that effectively ended the power of the river to flush sediment out into the bay. The result of all these interventions was increased natural and manmade growth of the “polders,” the salt marshes that invade the bay and provide valuable agricultural land, together with accelerated sedimentation around the Mount itself. In modern times, the foundations of Mont-Saint-Michel were buried under two meters of sand that had built up since the nineteenth century - the entire sedimentological system of the inner bay was changed – dramatically illustrated in the two maps below showing the locations and changes to the river channels before and after these “management” initiatives.

 

The project to restore as far as possible the nature of the bay has been comprehensive, evaluating hydrology and sedimentology, environmental factors, and the influence on tourism. It is is superbly described at http://www.projetmontsaintmichel.fr/en/, much of the site available in English. The sedimentology modelling alone was a four-year study, a key part of which was the construction of a huge physical model capable of reproducing and predicting sand dynamics (see photos below). The model covers 900 square meters (almost 10,000 square feet) and was used to model 45 years of annual cycles in the bay without further intervention and with a range of restoration options (see my earlier post for a natural version of such a laboratory and references to other such modelling projects).

Over a recent period of forty years, 27 million cubic meters of sediment have been added to the bay and a thousand hectares (four square miles) of new vegetated land encroached into it. The model demonstrated that, by 2042, the marine environment of the inner bay around the Mount would vanish completely. Nothing can stop this happening eventually, but it can be slowed down. There are two keys to delaying the inevitable: remove the causeway and restore the hydraulic power of the Couesnon river to flush sediment out of the inner bay. The projects to accomplish both of these, together with all the associated works, were commenced in 2006 and are due to be completed by 2015. The causeway will be replaced by a simple access structure raised above the sea floor to permit natural flow and sediment movement, and well away from the mouth of the Couesnon. The gigantic parking lot will be demolished, to be naturally replaced by 15 hectares of sand. The access to the Mount will be severed under the highest spring tides of the year, restoring the symbolic isolation of Mont-Saint-Michel.

On the river itself, a barrage has been constructed that will allow the waters of each high tide to be stored behind it and then released, flushing sediment out into the bay – the hydraulic power of the river will be restored, even enhanced. The barrage, shown under construction below, has just been commissioned. The river below the barrage will also be dredged; the product of the dredging, 1.25 million cubic meters of salty sand full of shell fragments that is locally known as “tangue” will be used, as it has been for centuries, for local agriculture and polder maintenance. Importantly, the modelling demonstrated the importance of the river draining in two channels out into the bay, so a structure to split the flow downstream from the barrage opening will be constructed.

The effects of the project will be gradual, but in thirty years or so it is anticipated that the mean level of the sea bed around Mont-Saint-Michel will be 70 centimeters lower than today, millions of cubic meters of sediment will have been removed (much of it quite quickly), and the sea will have recaptured 50 hectares of today’s land. The scene today, below left, will have evolved into something like the view on the right. This is an extraordinary and ambitious project, a fascinating experiment in large-scale restoration of the natural landscape.

The site linked above is the source of much of the information and illustrations for this post, and includes a thorough documentation of the sedimentological work, downloadable as a pdf file here; it’s in French, but superbly illustrated. I have copied below a map of the main components of the whole Mont-Saint-Michel project for reference and further detail.

The "Dieu Protège"

In the previous post I cited the joys of serendipity, and they continue. Travelling down the west coast of Brittany, we were attracted to the old fishing harbour of Douanenez and Le Port-Musée there, a boat museum. This is a museum with a difference - not only are there indoor exhibits of everything from coracles to twentieth-century fishing vessels, but outside, in the harbour, is moored an extraordinary collection of boats that have been painstakingly collected and restored, and around which you can wander at will, clambering up and down ladders, trying out the wheelhouses. Serendipity arose when I discovered that, among these boats is Le Dieu Protège, a gabare sablière - a sand transport.

To the north of Douanenez is Brest, a major port for centuries and the focus of extreme conflict during the Second World War. Used as one of the extensive U-Boat bases and a harbour for German battleships, including the Scharnhorst and the Gneisenau, Brest was the target of constant allied bombing raids. In 1944 the battle for its liberation was vicious, and by the end of the war literally only a handful of buildings remained standing in the city, including significant sections of the mediaeval harbour fortifications. The images below show Brest harbour before and after the war.

The post-war reconstruction of Brest took years and required huge volumes of concrete that, in turn, demanded huge volumes of sand. Le Dieu Protège ("God Protects") was built in 1951 specifically to excavate, transport, and deliver sand for the rebuilding of the city - the sand trade was thriving. It seems that the first owner had proposed various names to the authorities, all of them refused because of other boats already claiming them. He came up with the name in memory of a small boat sunk in the treacherous coastal waters after delivering the church bell to the island of Ouissant (presumably the protection extended only to the bell....). The boat is 23 meters long, has a capacity of 150 tons of sand, and has double wooden hulls to protect against the abrasive effects of its cargo and the stresses of large volumes of sand being dropped into its hold. It was equipped with sails (below, right), but was motorised and was fitted with a substantial mechanical bucket grab (named in French, le crapaud - the toad).

Until this innovation, sand extraction had taken place by beaching the boat and loading the cargo by hand and shovel, assisted, if on the beach, by horses, as shown in the delightful model at the museum (above, left). The bucket could hold 70 cubic meters of sand at a time, and around two hundred bucketsful of sand needed to be hauled up to fill the hold. [Although quoted from the museum documentation, the bucket capacity seems excessive and the numbers are inconsistent - see comment below]. The remaining requirement was a water pump to remove the 15 tons of water that drained out of every cargo. Four anchors would secure her to an offshore sandbank while she worked.

Le Dieu Protège's working life continued for thirty-five years - her last job was dredging the local harbours. She was purchased by the museum in1990 and restored - and, as the photo below shows, she is being further restored today.

 

Go down the steep ladder to the hold today (now empty of sand), and there are fascinating movies being shown of the sablière at work in her heyday. I have taken the liberty of extracting a few still images from these:

 

My thanks to a superb museum - http://www.port-musee.org. There's much, much more to see than Le Dieu Protège, so, if you're in Brittany, take time for a visit.

And I'll leave you with a picture of le sablier at the helm of a gabare sablière  - it's a great place for kids.

 

Nature's patterns, fractal landscapes, and a beach laboratory

The termination of the seemingly endless stretch of beach and dunes that forms the Atlantic boundary of the Bordeaux region is Cap Ferret, a constantly changing and migrating spit of sand. This guards the entrance to the Bay of Arcachon, a sweeping tidal expanse of sand and mud, and a source of excellent oysters.

My arrival at Cap Ferret was serendipitous in two ways: it stopped raining and the tide was well on the way out (a condition referred to ominously in French as le reflux). I walked past diverse attempts to stabilise the dunes, and warning signs on the risks of "collapsing" and dangerous beaches in the no-access area towards the end of the cape (with suitable polite apologies for the inconvenience), and came upon an extraordinary sight.  

The massive and relentless movement of sand around the cape was reflected in a miniature landscape of shifting designs left behind by the tide and actively sculpted by the water draining down the beach.

A complex pattern of larger sand waves contained within it ripples, valleys, canyons, meanders, deltas, braided channels, point bars, channel bars - lilliputian river systems, actively eroding, transporting, and carving their topography before my eyes. The sand contained a significant proportion of dark minerals, heavier than the quartz grains, which, like the granular segregation of placer deposits, etched out the details and designs of these landscapes. Natural processes operating without heed to scale, features a meter across that could be images from Google Earth - truly fractal designs. I lost track of how many photographs I took - every few steps there was something different and wondrous going on (including the sensation of being sucked into almost quicksand). Endless complexity and beauty: here, just a sampling.

And it struck me that I was in the midst of a vast natural sedimentological laboratory, an ideal and exciting place to examine the complexity and dynamism of natural processes and structures, in the same way that Steve Gough does at Little River Research and Design, or like the model systems at the University of Minnesota's St. Anthony Falls Laboratory. Except that Cap Ferret is much, much, bigger and the landscapes change with every reflux.

Climate change - or not

I was much looking forward to encountering Mont-Saint-Michel, not from the point of view of a pilgrim, or even a namesake, but simply to observe a sedimentary drama that is in the process of being rewritten, restaged, restored. So, on arriving in the delightful town of Avranches (and having, after visiting Utah Beach, admired the prominent memorial in Avranches to General Patton, and the sentiment that lay behind its existence), I ventured to the botanical gardens from where a panoramic view over the bay was promised. What I had forgotten was that, by simply crossing the English Channel, I had not left English weather behind - the word "perturbation" is one of those that is the same in French and English, and, when it comes to the weather, is highly descriptive. Soaked and dripping, I arrived at the viewpoint, which had, in addition to redundant telescopes, one of those very informative panoramic displays whose informative value depends entirely on good visibility. The image above is what I saw, the image below what I was hoping to see. Things did later improve somewhat - the rain paused; I'm not talking of bursts of glorious sunshine but I did dry out a bit. The story will continue .....

On the road - le sablier sur la route

 

I'm off for a bit of an excursion, wandering, meandering, and otherwise peregrinating through Brittany and then down the Atlantic coast of France - something I've wanted to do for a while. There will therefore be some uncertainty around when and whence the next post. But coasts are fertile environments for arenaceous musings, and who knows what will catch the interest of your faithful blogger. I'll be starting at Mont-Saint-Michel, pictured above, a World Heritage Site that is the focus of an ambitious sedimentological restoration project - so there's grist for my mill already. Then Brittany: ancient geology, strange linguistic heritage, and sandy coves; and my birthday (details withheld) at the westernmost point on mainland France.

 

Turning left, the plan takes in Les Sables d'Olonne (un "côte sauvage" but also a major port where the Vendée Globe round-the-world single-handed yacht race begins and ends) and onwards to the estuary of the Gironde and the bay of Arcachon.

 

The white expanse at the top right of the image above is the Dune of Pyla, the largest in Europe and active. I think I might drop by. And then what? Well, the Gironde runs through a region called Bordeaux - I'll figure out something ......

 

[Mont-Saint-Michel image by Uwe Küchler, GNU Free Documentation License, http://commons.wikimedia.org/wiki/File:Mont_st_michel_aerial.jpg; Dune du Pyla photo from http://kitusai.blog.lemonde.fr/2007/01/21/dune-de-pyla/]

Sand as a Problem

This blog celebrates sand and its wonders, its intersections with our lives and the processes of our planet, but, let's face it, sometimes sand can be at best a nuisance and at worst the cause of major problems. As the poet Robert Service is reported to have written, "It isn't the mountain ahead that wears you out - it's the grain of sand in your shoe." Sand gets everywhere and penetrates everything, and sand and mechanisms of any kind do not mix well - "sand in the gears." Sand and electronic devices represent a particularly lethal cocktail, as I found out to my frustration. At one point during our Western Desert trip, attempting to follow in the footsteps (and tire tracks) of Ralph Bagnold (the man who figured out how deserts work), we were struggling to reconcile the different scales of the original maps with the ones we had, and to convert locations to GPS way-points. I helpfully remembered that, like all mobile (cell) phones, mine had a calculator, and I extracted it from safekeeping in my backpack. The necessary calculations were made, and we were off - but I neglected to properly re-secure my phone, leaving it vulnerable to the Sahara and the two sandstorms we subsequently enjoyed. By the time, ten days later, we arrived at the relatively civilised environment of an oasis and I was ready to phone home, I discovered that the keys would only respond occasionally to even severe pounding, and that the process was accompanied by that faint crunching, gritty, sound. Penetration had clearly occurred (I believe this was also when granular materials found their way into my knee joints). And to think that I had, at one point, been contemplating bringing a notebook computer.

When I was back in the UK, I took the phone to the shop where I had bought it, and explained that it had ceased to work properly, muttering about "ever since I passed a building site on a windy day" - it was repaired under the warranty. But now I have made an interesting discovery. I was reading the Geographical Magazine, our equivalent of the National Geographic, published by the Royal Geographical Society (I was particularly enjoying this issue because they reviewed my book). In addition to the review, there were a number of articles of minor interest, but one caught my attention. Written by Bernard Sèbe, it described the "gear he took on his camel-dependent journeys across the Sahara." In addition to advice on choosing your camels, and whether to hire or buy them, he listed a "ruggedised computer" that "should resist sandstorms and maybe even a camel stampede. It's as heavy in your hands as it is on your chequebook, but you'll no longer worry about grains of sand destroying your machine." It turns out that, of course primarily targeted at the military market, there are extremely tough laptops and notebooks out there, and I looked up the recommended manufacturer on the internet. Below is a "Toughnote" ruggedised computer, that I have tastefully placed in the Saharan sand, next to some fossilised wood. I'm pleased to say that these are made in the UK by a British company; the website exhorts you to "get a quote" ...... They make ruggedised laptops, notebooks, hand-helds and peripherals; I was tempted to title this post "any computer port in a storm." Sorry......

 And then, in an entirely different publication, I was yesterday treated to the image below, accompanied by the words

Feeling the heat? Are sand management problems impacting your bottom line?

Well, I thought, the book sales could be better, perhaps.... and then I realised that it was a different kind of sand problem. Let's, for a moment, put aside the issues about fossil fuel use, accept that, at least for today, oil and gas are quite useful for our way of life, and consider the truly remarkable technology and engineering that goes into extracting them. These critical resources do not occur in subsurface lakes, rivers, or caverns; they inhabit the microscopic holes in buried rock, very often the spaces between sand grains. Any rock that, like a sponge, is sufficiently porous to hold significant amounts of water or hydrocarbons is called a reservoir. The quality of a reservoir for any fluid, whether it's water, oil, or gas, depends on its porosity and permeability—how much fluid it can contain and how efficiently it will give it up. These characteristics depend on the nature of the sand grains, the spaces between them, and the connections between the spaces. But however porous and permeable a reservoir may be, it will never give up all its fluid; the same surface tension effects that help build sand castles hold water or oil onto and between the grains and keep more in the rock than can be extracted. If the reservoir is under natural pressure, then, once a hole is drilled into it, the fluid will flow to the surface of its own accord; otherwise it has to be pumped. The world’s largest accumulations of oil and gas are found in the spaces between sand grains, as are countless numbers of smaller fields. In Alaska, Saudi Arabia, Russia, and elsewhere, the reservoirs are made of sandstone. The reservoirs at the giant Prudhoe Bay field in Alaska are Permian and Triassic sandstones, deposited in shallow marine environments, deltas, and the beds of braided and meandering rivers. In the early days, it was thought that the sand grains and surface tension would only allow 40 percent of the oil in the reservoir to be extracted; technology has now increased that to almost 60 percent—but that’s still a lot of oil left underground.

The engineering that goes into producing the fluids from a reservoir kilometers below the surface is very sophisticated, but one of the common problems it has to deal with is that the fluid is not the only thing produced at the surface. Very commonly, the sand grains are not completely glued together, the reservoir rock is not thoroughly lithified, and, along with the oil comes sand. The photo at right is an example where the engineering has not been as clever as it might be, and a pile of sand has accumulated under the drilling rig. But this is rare - sand management applies all kinds of technology for preventing the sand ever entering the well bore, and screening it out when it does - after all, the last thing needed is for sand and machinery to mix. And so the image above is from an advertisement for a high-tech geosciences and engineering consulting company that specialises, among many other activities, in controlling and managing sand production. But this managing and controlling is expensive, hence the allusion to the bottom line.

So yes, sand can cause problems, some of them major - but then none of us are perfect.

[Sand management images from the Senergy World website. For ruggedised kit, see http://www.terralogic.co.uk/. This is not a commercial for either!]

   

Sand contributes to the economic stimulus

As a young kid, I was blessed with six great-aunts, who lived together in two groups of three, one group in Nottingham and the other in the East End of London. Visiting either somewhat eccentric group was always a treat for me, their rambling old houses filled with strange (but still memorable) aromas, exotic Victorian decoration, and sundry mysteries (including the outside toilets). The London group consisted of two spinsters and a widow plus cats and, generally, a lodger priest whom I tended to avoid. I loved visiting, particularly if my great-aunt Grace (my favourite) had made bread pudding, still the best I have ever tasted. But a culinary mystery, and a genuine piece of kitchen magic, was when she preserved eggs. These were still the post-war days when eggs were precious, and, in any case, old habits die hard. The eggs were immersed in this strange substance, which began as a viscous fluid, diluted for the purpose. "It's waterglass, dear," my great-aunt would explain; this struck me as being very descriptive, since it looked like liquid glass, but little did I realise how accurate the name was.

Waterglass is sodium silicate, or, more strictly, sodium metasilicate, Na₂SiO₃, a chemical that easily absorbs water, starting off in my Great-Aunt's kitchen as a powder, and hydrating into a viscous fluid and then a true liquid. It seems that it penetrated the shell of an egg, sealing off the porosity and thereby preventing access by bacteria; eggs preserved this way could be kept for months. Waterglass showed up later in my life in various guises, including as an  sealant for various leaking bits of my early cars - it was particularly effective for a hole in the exhaust pipe, the heat of which would solidify a patch soaked in sodium silicate into a hard seal. The irony today is that sodium silicate is hitting the headlines not as a means of mending your car, but of completely crippling it. As a recent USGS news item reported:

The Nitty Gritty of Cash for Clunkers

The government-run “Cash for Clunkers” did much more than just stimulate the economy and raise awareness of carbon emissions. It also caused demand for a little known and little used mineral compound called sodium silicate. The program required that buyers of clunkers immediately kill the engine of the car — a task most efficiently done by running the engine with sodium silicate. When a vehicle runs for a few minutes with this compound in place of engine oil, the engine seizes, and it cannot be reused. The other parts of the vehicles, however, can be recycled.

The "Cash for Clunkers" project has caused a massive demand for an obscure mineral. As the Wall Street Journal reported a few days ago, under the headline "The Killer App for Clunkers Breathes Fresh Life into 'Liquid Glass'," the "designated agent of death for cars" suddenly became a highly sought-after commodity. One chemical supplier

ordered enormous supplies and purchased prime space on Google, so that his company popped up in searches for sodium silicate. Last week, he sold 4,600 gallons of it, and the rush is continuing. "We're working 16 hour days, and we've got friends and family helping out filling orders."

So where does all this magic, lethal, mineral come from? As the USGS reports, "Sodium silicate, the only soluble silica compound, is made by fusing two industrial minerals, high-purity silica sand with soda ash." Sand to the rescue of the global economy!

"Soda ash" is sodium carbonate, the naturally occurring form of which is the mineral natron. And so we come back to glass - "the stone that flows." The satisfying story that Pliny the Elder tells of the origin of glass is, sadly, apocryphal, but it's entertaining and illustrative of the process. He describes, in his Natural History, how Phoenician traders with a cargo of natron, perhaps from the desert lakes of the Sahara, had put in for the night on the eastern Mediterranean coast. Unable to find adequate rocks to support their cooking pots over the fire, they resorted to using some blocks of their cargo. Whatever their dinner recipe was, they had unwittingly assembled the ingredients for glass: the soda lowered the melting point of the beach sand, and out of the fire flowed streams of translucent liquid. It's a nice fairy-tale, but with an important point: to melt sand on its own requires temperatures in excess of 1,600°C (2,900°F), far beyond the capability of traditional wood-burning furnaces. The melting point has to be lowered to a practicable temperature, and this is where the natron plays its critical part in the story. Soda has a much lower melting point than sand and acts as a flux, an additive that makes silica sand meltable at realistically achievable temperatures.

But of course what is being made here is sodium silicate - waterglass, a product soluble in water, which is hardly desirable in making a wine glass or a window. To stabilize the glass, calcium in the form of powdered limestone has to be added, and it is entirely possible that this was first discovered by accident through using silica sand that also contained fragments of seashells. The resulting concoction is soda-lime glass, the everyday kind of glass that has been used for the last three thousand years.

Sodium silicate, while it is a somewhat obscure chemical, does, nevertheless, continue to be used in a number of different ways. It's still used as an adhesive and sealant, in treatments for timber, concrete, and water, and as a fire protection. Perhaps its most important use today is in the manufacture of synthetic zeolites, clever minerals that have a whole host of uses. It's also used in Magic Rocks. Although that name hadn't been yet coined when I was a kid, this was the other incarnation of sodium silicate that I encountered. In the days when chemistry sets really were chemistry sets, with ingredients capable of causing real domestic havoc and incurring real maternal wrath, one of the more benign but common chemicals provided was waterglass. With this, and some compounds of different metals, a "chemical garden" could be created. Put a layer of sand at the bottom of a glass container, add waterglass dissolved in water, and then chunks (the magic rocks) of different metal compounds, generally nitrates or chlorides of manganese, cobalt, copper, iron, and watch your garden grow. The metals combine with the waterglass to form insoluble metal silicates that precipitate and grow upwards to form exotic plant-like structures. The colour depends on the metal, just as the colour of glass depends on the metal added. The phenomenon was discovered in the 17th century, when waterglass was known as "oil of sand" and played a common role in alchemy. It would seem that Sir Isaac Newton, in his experiments in the transition between alchemy and "chymistry," created magic gardens in oil of sand, the organic appearance of the structures confirming the old alchemical idea that metals have a kind of life and can "vegetate".

It's sort of fun where a simple story of disabling clunker engines can get to - my great-aunt Grace would be pleased.

Arenophile genetics, Jedi, and another submarine canyon

Geology doesn't exactly run in my family. My parents weren't geologists, I married someone to whom dip and strike will eternally remain an arcane concept, and neither of my kids are geologists. Geology has, however, touched, so to speak, them all, and rubbed off most on my daughter. She's the one who has always loved the outdoors and adventures. She became, involuntarily, intimately familiar with the major faults of the Atlas Mountains during our trek there together when she turned fifteen (she was, according to our guide, the youngest western female to climb Jebel Toubkal, the highest mountain in North Africa). And her recent activities would suggest that arenophilia may be genetic.

Earlier this summer, on a well-deserved vacation/holiday in southern California, where was her first  destination after arrival? The Imperial Sand Dunes - in the distinctly uncomfortable temperatures of July. The area, also known as the Algodones Dunes, is the largest landscape of sand in California, covering its intersection with Mexico and Arizona. And there she collected, secured, and labelled in detail, plastic bags of sand for her arenophile father - mad dogs, Englishmen, and their daughters. En route, she came across the strange sight of a small group of Star Wars figures, for it was here, in 1982, that a massive set for Return of the Jedi was constructed. And, in the same way that the archaeology of the Ten Commandments has become an attraction of the dunes at Oceano (see my earlier post), so the Imperial dunes are a place of pilgrimage for devoted Star Wars fans who re-enact and dig. This phenomenon was covered by a highly entertaining article earlier this year in Harper's Magazine, " Raiders of the lost R2: Excavating in a galaxy far, far away." As one of the pilgrims is quoted as saying, “As a kid, you can only go so far playing with action figures. As an adult, you don’t play with action figures anymore. You become the action figure.”

Escaping from the desert heat, my daughter then sought refuge on the beaches around San Diego - and continued to assiduously collect and catalogue sands for her deranged father. And, in the process, she made an interesting and geologically significant, observation. The beaches of La Jolla Shores and La Jolla Cove are immediately next to each other, the Cove being located in the rocky headland that juts out at the southern end of the Shores. Yet their sands are dramatically, visibly, different. Between your toes on La Jolla shores is a fine sand (upper image, below), quartz grains shot through with dark rock and mineral fragments; at the Cove (lower image, same magnifications), the sand is coarse but poorly sorted, made up of a wide range of grain sizes, with shell fragments clearly visible, along with dark minerals, oranges, pinks, and purples. This contrast seem bizarre - until we look at the geology and what's going on along this stretch of coast.

 

First, the setting of the Cove is distinctive. It's backed by sandstone cliffs shown in the photo, weathering and eroding away at a rapid rate. The sandstone was formed around 70 million years ago as the tectonic turbulence that has long characterised southern California raised the Sierra Nevada mountains. The Sierras were formed as a series of gigantic bodies of originally molten rock, plutons, emplaced deep within the earth's crust, but rapidly jacked up to the surface by the tectonic turbulence. Once there, they were worn away by the relentless forces of weathering and erosion, the sedimentary debris forming the sandstones of La Jolla Cove. Now those sandstones are themselves providing much of the sedimentary debris for this segment of the coast - you can almost sense the sand pouring off the lower part of the cliff onto the beach - local ingredients.

 

Second, and even more importantly, if you were to pull the plug in order to see the landscape beneath the ocean along this stretch of shoreline, then an extraordinary topography would emerge. Just a few hundred yards offshore from the southern end of La Jolla Shores is the head of a huge submarine canyon. Similar to the Monterey Canyon further north (that I wrote about here), this feature is deep and sinuous, winding its way far out across the Pacific Ocean floor. And it's a sand-sucker - any sand that is carried down the coast along La Jolla Shores from the north is inevitably and irrevocably swept down the canyon and out to sea. The La Jolla canyon forms the boundary between two segments of coastline that are distinct with respect to their sedimentary processes and the transport and deposition of sand - two different littoral cells (see my "Beach Nourishment and Sediment Budgets" post). 

 

No wonder the sands of the Shores and the Cove are so different. This is a dynamic stretch of coast and the submarine canyon has long been a laboratory in which we have learned something of the dramas that are played out in these landscapes. And it's perhaps no wonder that one of the world's foremost centres of oceanographic research, Scripps Institution of Oceanography, is located (on land) at the head of the canyon.

[The image of the canyon, along with many others, can be found at http://walrus.wr.usgs.gov/pacmaps/sd-index.html; Scripps put out a news release a couple of years ago that describes some fascinating research on sand movements and budgets along this part of the coast: http://scrippsnews.ucsd.edu/Releases/?releaseID=785; the department of geology at San Diego State University have, brilliantly, put together Google Maps overlays of the geological maps: http://www.geology.sdsu.edu/kmlgeology/kmz/la_jolla/la_jolla.html; and see the comprehensive study of California's coastal processes and littoral cells, Littoral Cells, Sand Budgets, and Beaches: Understanding California’s Shoreline. The photo of the Star Wars set is from http://www.desertusa.com/sandhills/du_sh_star.html. And, finally, thanks to my daughter for her genetic peculiarity and for prompting the basis for this story.]