The local basalt is of excellent quality for road building, railway track ballast, harbour works, airport runways and other construction work requiring a strong sharp-edged stone. Crushed stone and individual larger stones weighing 5 tonnes or more were exported by ship from early in the 19th century. “Stone Boats” were a regular sight in the harbour up to the 1970’s when the trade ceased. Throughout the 19th century stones would have been brought to the harbour by horse and cart and manually transhipped, in later years using small steam powered cranes, into the holds of the waiting ships. Such methods could take up to two days of hard dirty work to load 500 tons (508,023 kg) of stone. This trade increased to such as an extent that construction of four large silos was started in 1912 on the North Pier of the harbour.
These silos, known locally as the “stone bins” were built in reinforced concrete and held a total of 500 tons (508,023 kg) of crushed stone – 125 tons (113,797 kg) in each silo. Work was completed in 1914 just before the start of the First World War. Stone arriving at the quay side could now be dumped in a sunken hopper at the rear of the silos from which it was lifted by a chain bucket conveyor system to the top of each silos and dropped in. Ships moored alongside the silos were loaded by metal chutes which were lowered from each silo and down which the stone flowed into the ship’s hold. The first ship to use the new silos was the Wheatlands and the 500 tons (508,023 kg) were loaded in 40 minutes!
The bins ceased operation in the 1970’s as competition from enlarged road carriers and bigger and better equipped ships from modernised ports rendered them obsolete. Demolition of the silos proved a difficult task as they had been strongly and solidly built. Their removal greatly altered the harbour landscape as their towering bulk had been a feature for more than sixty years. It has been argued that they should have been preserved as a wonderful example of industrial heritage, almost unique in Europe.