Cobbles occur in various sizes and colours. Cobble structures are normally found near to the coast or in towns near rivers, because cobbles are smoothed and rounded by the action of water. In the 18th and 19th Centuries cobbles were often used as ballast for grain and wool barges. The ballast would be offloaded to make room for the new cargo, depositing readymade building materials on the quayside. Depending on availability, cobbles may be laid either randomly or in courses.