Where does road salt come from?

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From here, beneath the ground in Winsford, Cheshire...

With heavy snow falls so far this week, and more on the way, supplies of the salt used to grit roads and footpaths are running low. Where does the salt scattered by gritters come from?

De-icing salt - or grit - is nothing like the white grains we use to season food. Made from crushed rock salt carved out of underground mines, it is typically brownish in colour and resembles gravel.

The salt used to melt ice and add traction on the UK's snow-covered byways comes not from the sea but from three main mines - the Salt Union's Winsford Rock Salt Mine in Cheshire, Cleveland Potash in Teesside and the Irish Salt Mining and Exploration Company in County Antrim.

THE ANSWER De-icing salt is rock salt carved from mines deep underground Mine deposits formed as anicent bodies of salt water dried up How do they know when to grit?

These mines were formed millions of years ago when the UK and Ireland were covered by inland seas. Over time, the seawater evaporated, leaving vast salty deposits that were gradually covered over. Some are 100m deep, others well over 1.6km - a mile - underground.

Today rock salt is extracted by machines known as continuous miners fitted with rotating steel cutting picks which grind salt from the walls of these vast cathedral-like spaces.

... to the gritting lorries

"The salt is then carried away from the cutting surface by conveyor belts to be crushed and treated further," says Salt Union spokeswoman Katie Moffat. It is treated with anti-caking agent, then put into storage to await transportation to gritting depots by lorry or rail.

Those who work the salt mines are trained engineers who keep the machinery running - a far cry from the pickaxes and buckets used in the mine's early days in the 1800s, when rock salt was primarily used for salt licks - blocks of salt - for animals.

Deep storage

Rock salt is now used for winter highway maintenance as salt lowers the freezing point of water to below zero Centigrade - how low depends on the concentration of salt to water.

A miners' tea break in the 1920s

This means when salt is spread over a road or a footpath, it either melts the snow and ice as it dissolves, or helps prevent ice forming.

After heavy snow throughout this week and more forecast to fall, de-icing salt has been in such demand that council stocks are running low. And to restock when the roads are slick with ice can be problematic, so supplies are being rationed. Meanwhile, production has been stepped up at Salt Union's mine which has been working 24 hours a day since the beginning of January.

But as well as drilling salt for the nation's byways, Salt Union has also found a use for the spaces left over by mining - storing important documents.

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"A salt mine is clean and dry, with huge caverns carved over time, leaving giant pillars of rock salt for stability," says Ms Moffat. "With low humidity, no water and no UV light, it's perfect for archive document storage."

As well as storage rooms, the spaces formed by extraction also form road-sized pathways for the miners and mining equipment to move around between cutting surfaces.

The Salt Association, the trade body for producers, estimates the UK's salt mines to have about 225km (140 miles) of tunnels - almost as long as the M5 motorway. Which is itself in need of de-icing after Thursday's blizzards.