The shifting Solway sands

Testing thixotropy (photo: Ann Lingard)

Take three Year 9 boys, part of a group from Settlebeck School that the Head of Science and I have brought to Mawbray to show them the "science of the shore": science is boring and un-cool - can we find something to enthuse them? They have been warned beforehand to wear wellies, so they wear trainers. We tell them about "thixotropy", partly because it's a good word, the name for when sand changes state from firm to soft and sloppy - and so the boys stand in one place and stomp their feet up and down like seagulls "paddling" for earthworms in a field. Who can sink deepest? Feet and trainers disappear ...

Wind-sculpted sand on the upper shore (photo: Ann Lingard)

Probably the most obvious characteristic of the southern shores and sand-banks of the Upper Solway is that they are constantly changing and shape-shifting, on a scale ranging from landscape to hand's-span to microscopic. This shape-shifting has influenced everything that has happened here from geological to contemporary times, and is what makes the Solway Firth so unique.

Every time I visit the shore it looks different, as the tides and wind constantly sculpt and re-work the sandscape, whether between the tidemarks or higher up on the edges of the dunes.

'Sand waves' created by longshore drift (photo: Ann Lingard)

Longshore drift, the direction in which the current travels up the coast, brings animals and objects from further South -- including all kinds of unexpected objects hurled out to sea by the River Derwent during the November 2009 floods. Sometimes the current creates large "sand-waves" to mark its progress, the hollows between them trapping pools of water.

There is continual loss and gain, wind and water eroding and moving sand from place to place, revealing rocky scars or Roman burial grounds, burying salt-marsh and fish-traps, creating new sand-banks or carving new channels. Rivers meander into the Firth through shifting paths, a port silts up and becomes unusable; trawlers and cargo vessels must navigate with care.

Grune point, and the River Wampool entering the Solway
(my thanks to Simon Ledingham for these aerial photos)

But it is in the intertidal region that the changes shift to the scale where you can reach out and touch them, or cover them with a footstep -- sand-ripples, their patterns varying from metre to metre, locked into shape until the tide returns to re-mould them. Chevrons, fish-scales, dimples: there doesn't seem to be any logical reason why the ripples here should have a rounded profile while the ripples over there are taller and sharper; widely-spaced, or close together, or with tiny gullies down their sides.

Ripple patterns in the sand (photos: Ann Lingard)

Do the ripple-patterns form when the tide comes in or when it goes out? Do they form and reform all the time they are submerged, according to the ebb and flow of the water? The shore is a good place for stimulating simple questions. I thought it would be easy to find an answer, but was intrigued to discover that nobody seemed to know. I asked physicist Philip Ball who has written three excellent books on patterns in nature, and geologist Michael Welland who has written both a fascinating book called 'Sand, a journey through science and the imagination' and a blog, Through the Sandglass. Michael and Philip agree that science has only found partial answers at best.

'Land puckering between grey-green wheals': Acrylic and mixed media on canvas. (My thanks to Joe Dias for allowing me to use this image of his work)

Michael Welland had been here to talk to our Cockermouth Café Scientifique, and we had all enjoyed a very entertaining evening of "party tricks", his practical demonstrations of the strange properties of sand grains. So I was pleased to be able to take him down to Beckfoot the following morning - here, especially, some of the ripple patterns are spectacularly contrasty, the valleys between the ridges accentuated with a fine black sediment of powdered mussel shell, thrown up from the vast mussel beds lower down the shore.

Marram grass and banded snails on the dunes (photo: Ann Lingard)

At Beckfoot and Mawbray Banks there is also the added delight of walking on the dunes, on sand that has been piled and sculpted by the "aeolian" effect of the wind. You can climb up and walk along the newest layer, bound by marram grass, silvery sea-holly and other dune vegetation - the AONB's Nature Reserve that is home to gentians, burnet and cinnabar moths, banded snails and natterjack toads.

The different layers in a sand-dune show its history (photo: Ann Lingard)

The seaward side of this nature reserve, part of the Solway Coast AONB, is being continuously eroded; the Solway's spring tides may rise and fall by as much as 10 metres, and if the barometric pressure is low and there is a strong North-West wind the waves pound high onto the shore, battering the free edges of the dunes so that large vegetated plates from their margins tumble onto the shore. But this erosion lets you look beneath the surface, at a cross-section that shows how the dunes have grown over thousands of years: their layers of pebbles, sand, dark organic earth and rotted vegetation reveal some of the Solway's changing history, its stories of raised beaches and fluctuations in sea-level.

As you can read in the chapter on the "Submerged Forest", the sea sometimes reveals other secrets too, washing away the sand that covers the peat and the trunks and branches of the boreal forest that once grew over the land that lay where the Firth is now.

Sand grains from shore and dune under the microscope (my thanks to Michael Welland for these photos)

If you take a box of muesli and shake it, the largest and heaviest pieces rise to the top, a phenomenon known as the "brazil nut effect". So it is with sand grains of different sizes - they sort themselves out according to size, through the stirring effect of wind or water. This agitation has other effects too, a weathering effect so marked that you can tell (with the help of a magnifying lens) whether the grains were deposited in a river-bed or on a dry plain.

As we wandered along the beach on that breezy day, Michael Welland took small samples of sand from the shore and from the dunes. A few days later he sent me photographs of the samples as seen through his microscope. The sample from the dune shows rounded and frosted quartz, typical of sand blown in the wind; it also contains grains of colourful metamorphic rock. In contrast, the sand from the beach is fine-grained and angular, the quartz and rock fragments sharp-edged and typical of water-borne sand. They all look like precious stones.

Sabellaria tubes (photo: Ann Lingard)

Sand as protection

Down near the bottom of the shore there is a strange landscape of sculpted rocks. But these are not geological specimens: touch them, use a hand-lens to examine them close-to, and you will see that they have been constructed from grains of sand. These extraordinary reefs are special to the Solway coast, and each mound is a twisted mass of tubes, each tube made by a worm called Sabellaria. Related to the earthworm and the lugworm, it has no eyes and only a primitive nervous system, and yet with its tentacles it selects minute sand-grains, picks them up, and cements them in place to create a hard protective dwelling.

A Lanice tube, constructed from sand and shell fragments, beside a patch of barnacles (photo: Ann Lingard)

Nearby in the pools, cruder tubes with apparently "hairy" crowns stick up rigidly from the sand, each built by a worm with the attractive Greek name Lanice - again, the worm selects and glues together grains and pieces of shell within a certain size range. But the Sabellaria landscape is not permanent: different factors cause the reefs to change their size and shape, and sometimes they vanish altogether.

Evidence of lugworm burrows in the sand (photo: Ann Lingard).

Sand mixed with varying amounts of water has strange properties, as sand-sculptors and sandcastle builders know. Sand grains that have settled through the water, like those in the intertidal zone on the beach, become well-organised and closely packed. When you walk along the beach, you compress and disrupt that orderly arrangement and, completely contrary to what you would expect, the spaces between the grains are forced to expand. Water drains into the gaps and your footstep now looks pale and dry. If you stamp up and down you'll enlarge the spaces between the grains, so that more and more water flows in and pushes the grains apart. It forms a quick-sand slurry - which may then seize up solid again to cement your ankles in place! But testing thixotropy (or "making cows' bellies", as someone who used to spend a lot of time in Morecambe Bay more picturesquely calls it) isn't only for the amusement of Year 9 lads, it has a practical use for many of the animals that live within the shore: lugworms, heart-urchins, bivalve shells - all wriggle their spines or shells or "shoulders", and sink their burrows into the softening sand.

The "right sand" - according to the BBC's Coast programme, eight parts of sand to one of water - for building upwards rather than burrowing downwards may have to be sought by trial and error, but you will surely be able to find somewhere on Allonby bay to build your sand-castles. And when the tide comes in and erodes your masterpiece, there's always the consolation of one of those famous ice-creams from Twentyman's shop nearby.

Copyright: Ann Lingard, October 2010

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