The process flow starts with the mining of the mineral. This is usually achieved by blasting, followed by rubble disposal. The usable stone is then transported to the preparation plant where the feed product is crushed using crushers or mills and,if necessary, classified using screens or sifters. With some exceptions, the final products are construction materials which are subdivided according to their grain size into: crushes sand, grit, crushed stone, railway ballast and water engineering stones. If necessary, these construction materials are further processed to manufacture (for example) concrete or asphalt. By contrast, it is true that in ❑ gravel works construction materials are both obtained and prepared on the some site; however in this case, a loose sediment deposit of gravel and sand is minded by excavation. Despite many similarities between a quarry and an opencast mining operation, ❑ mine is only generally used if ores, coal, salt or other usually high value materials are being removed.