Shells, Sand and Beaches

Ben DolphinOn one of my days off last week I went to the beach. That’s not unusual in itself, seeing as I live in Fife. We have 117 miles of coastline right here with glorious sandy beaches and wide open vistas.

But sometimes, when I know the sun will be shining and the skies will be blue, there’s nowhere I’d rather be than on the west coast with its irresistible mix of beaches, hills, rocks and islands.

My destination for the day was therefore the stretch of coastline between Arisaig and Morar. A long drive from Fife I admit, but as soon as I stepped out of the car and onto the sands at Traigh, all thoughts of the drive there (and indeed the drive back) evaporated.

It was Caribbean… kind of. Yes, there was that nagging, ‘bracing’ Scottish breeze but the sea was crystal clear, the sky was deep blue and the sand was… white. Not snow white, perhaps, but not far off, and if you kneel down and pick up a handful of that sand it’s easy to see why this beach (and others like it) gleam creamy white on a sunny day.

A handful of Traigh’s white sand reveals its true nature

A handful of Traigh’s white sand reveals its true nature

Nestled in your hand are tiny, irregularly shaped, mostly white objects. It’s not clear at first what they are, not until you note the occasional small, whole shells among them. Then you’re struck by just how much of the material in your hand is shell rather than the tiny particles of ‘sand’ we encounter on beaches elsewhere.

Can this white shell sand still be called ‘sand’, then?

What is sand?

‘Sand’ is the general word we use when describing the small particles of matter that make up the beach. Unsurprisingly, there are all manner of fussy measurements in microns and millimetres that dictate whether or not we can officially call it ‘sand’.

I’m quite sure most of us know sand when we see it or feel it but, without getting too technical, sand particles are bigger than silt but smaller than gravel. And I imagine most people can tell the difference between those without getting tweezers and a microscopic ruler out.

Where does sand come from?

While the source of Traigh’s sand might seem obvious, pinpointing exactly where the sand on a given beach comes from can be difficult. That’s not surprising, given the vast distances some of it might have travelled and the many variables at work, such as undersea currents, wave and wind action.

That said, we do know that the sand on our beaches can come from any of four general sources: the remains of rocks ground up by glaciers; the remains of rocks churned up in our rivers; cliff erosion on the coast; or the remains of dead sea creatures.

The first three of those are ‘mineral’ sources, which is to say they’re derived from rocks. Perhaps surprisingly, the majority of this actually comes from rocks inland rather than rocks out at sea.

During the ice ages whole mountain ranges were ground down by glaciers or were fractured by repeated freezing and thawing. When the ice began to melt and rivers began to flow, the fine remains of the hills were washed out to sea.

That process ended around 10,000 years ago but erosion of the landscape continues today, as our hills and rocks are continually being broken down by the weather (frost, wind, rain etc).

Weathered rocks and stones end up in rivers and, as they travel downstream, they are rolled, broken apart and worn down. This sorts the broken material into different sizes until the lighter and smaller particles, suspended in the turbulent water, ultimately reach the open sea.

We have beautiful beaches in Fife but the mineral-based sand can give them a very different colour to that of pure shell sand. This is Burntisland when the tide is out.

We have beautiful beaches in Fife but the mineral-based sand can give them a very different colour to that of pure shell sand. This is Burntisland when the tide is out.

From both glaciers and rivers, a vast amount of sand was (and is) ejected from our estuaries and out to sea, and this forms the bulk of our offshore sand supply. For the beaches between Arisaig and Morar, most of the mineral sand is presumed to have come from the glaciation that occurred in and around what is now Loch Morar.

This is further supplemented at the coast itself. Cliffs and shoreline are slowly but surely broken down by the power of the ocean, and sand from their rocks is later deposited on the shore if conditions allow.

The fourth of the sources is organic rather than mineral, being the remains of a bewildering number of sea creatures. Specifically, those with shells or an exoskeleton made of calcium carbonate, such as mussels, limpets, crabs, corals etc.

Just like the mineral sources, the dead shells are treated very roughly by the marine elements until they too are small enough to be officially called ‘sand’…..albeit sand made from an organic source rather than rock.

How does the sand get to the beach?

Whatever the source, all that sand has to go somewhere once it is ground down. It gets deposited on the permanently sea-covered floor below the low tide mark. It is then churned up and moved around the coastline by currents and wave action, but a number of variables have to come together for a beach to form.

In addition to a ready supply of sand the potential beach needs to have the right aspect on the coastline, a not-too-steep slope into the sea, and good strong wave action or tides etc. Essentially, it needs to be exposed to natural forces strong enough to bring sand up onto it from the seabed, but it also needs the right shape and physical properties so that it can not only receive, but also retain the sand when it comes ashore.

If that sounds complex, it is. The fact that only 470 miles of our 7500 mile long coastline is defined as ‘beach’ goes to show how relatively uncommon a process beach construction really is.

Scotland’s shell beaches

In the course of writing this piece I read a fair few studies of Scottish beaches, and was surprised to find that the answer to why some beaches end up with such high proportions of shells, was somewhat elusive.

Possible explanations were that the areas close to shell beaches have historically had high concentrations of marine organisms or, conversely, that they have perhaps had little or no input from any ‘mineral’ sources nearby. Perhaps there were no rivers or estuaries to deliver sand from inland.

Some of the beaches near Morar have very high shell content

Some of the beaches near Morar have very high shell content

Others ascribed the dominance of shells on beaches to extensive underwater rock platforms just offshore. Whatever the reasons, shell beaches occur all around Scotland to a greater or lesser extent, but without doubt the west coast and the Hebrides have the sealion’s share. Dumfries & Galloway, much of the North Sea coast and the Firth of Clyde have the fewest.

The proportion of organic sand vs mineral sand in beach composition varies greatly but some of the beaches on Tiree, Coll, Mull and parts of Orkney are almost entirely composed of shell sand. They have a beauty all their own and a select few like Camusdarach near Morar, or Luskentyre on Harris, are world famous because of it.

Shells and Crofters

Cast your mind back to your school days, learning about farming systems. Before you start yawning you might remember that all soil can be ranked on a scale of alkalinity and acidity, and all plants and crops have different tolerances on that scale.

Some are fussy, growing only on very acid soils or very alkali soils. Some are less choosy, growing pretty much anywhere. But they all have preferences and limitations, and whether or not they can grow in a given location is determined by the chemical composition of the soil.

Much of Britain, especially in the upland areas, is overlain by acidic soils that aren’t suitable for growing certain crops. Farmers needed to make their fields less acidic so that they could grow whatever they wanted.

Now cast your mind back to school lessons in geology, and remember that limestone is made from the compacted remains of billions of sea creatures that have been fused together by heat and pressure over millions of years.

Across the UK, farmers have historically quarried and pulverised limestone into a powder (lime) that could be applied to their fields. This is because the shells of sea creatures are lime-rich, meaning they contain high quantities of calcium carbonate, which is highly alkali. Applying lime to their fields made them less acidic and more conducive to farming certain crops.

However, in the northwest highlands and islands where limestone is relatively scarce, shell sand has always been a handy source of lime when none would otherwise be available.

Furthermore, Mother Nature has done most of the work for the farmers already. In dry conditions the wind picks the finer sand particles up and takes them inland. This forms dunes behind the beach but it also forms a constituent part of the soil where it settles along the immediate coast.

Thus the coastal fringes of the northwest highlands and islands have a wildly different character to that of the interior, just a few kilometres inland. You don’t have to go far inland before the soil gets very acidic and you encounter the familiar bleakness of grass, heather and bog.

But on the immediate coast with its lime-rich soil there is productive cultivation, and rich grazing to be had on grasses and wildflowers. This scene of the white sandy shell beaches backed by dunes and wildflowers forms the basis of the ‘machair’. The globally-rare and fragile habitat for which much of the northwest is justly famous.

White sandy beaches between Arisaig and Morar

White sandy beaches between Arisaig and Morar

Though the process of enriching the coastal strip with lime-rich sand has evolved naturally, it continues to be exploited and strictly managed by crofters who know full well how important a resource it is, both to them and to wildlife.

Today, it is a landscape treasured by residents and tourists alike, and it’s all thanks to the tiny battered remains of untold billions of sea creatures.

The following routes visit particularly fine shell beaches:
Claigan coral beaches, Isle of Skye
Culduie coral beaches, Applecross
Plockton coral beach

Here’s a Walkhighlands’ pick of a dozen of Scotland’s best sandy beaches:
Camusdarach beach, near Arisaig
Sandwood Bay, Sutherland
Luskentyre Beach, Harris
Traigh Mhor, Lewis
Gaineamh Mhor, Gairloch
St Combs, Aberdeenshire
Gullane Bay, East Lothian
Kiloran Bay, Colonsay
Machir Bay, Islay
Calgary Bay, Mull
Hough Bay, Tiree
Coilleag a’Phrionnsa, Eriskay

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