They are being added to each year as rock salt extraction continues.
It is, rightly, a proud town, famous for its rock salt mines; and it is now dependent on many small and growing businesses.
But to spread rock salt by itself is most injurious to mechanical appliances.
The grit spreaders had gone 5,928 km, chucking out more than 1,220 tonnes of rock salt.
The cavity, which has been specially washed out of rock salt, is at a depth of 1,300 ft.
There is no doubt that brine is simply rock salt dissolved in water.
The council said that the gritters had been out, and they had, spreading 1,220 tonnes of rock salt.
The main cause of chloride attack on bridges is rock salt which is used for winter gritting.
