BBC NI showed a documentary about Belfast Lough last night, where they visited a mine near Kilroot which produces all the Rock Salt for gritting our roads on days such as today and also export the salt by the ship load to the east coast of America aswell as the rest of the UK.
So where did the salt come from?
Explanation via the BBC living history website:
http://www.bbc.co.uk/northernireland/livingworld/naturalhistory/walk/walk1.shtml
The presence of gypsum and rock salt below the surface (the salt is commercially mined at nearby Kilroot) shows that, although the land was dry, the sea must have sometimes flooded what would have been a low-lying area.

It was very hot and any standing seawater evaporated very quickly leaving the salt deposits behind.