Derbyshire is rich in natural mineral resources such as lead, iron, coal, and limestone, which have been exploited over a long period. Lead, for example, has been mined since Roman times. The limestone outcrops in the central area led to the establishment of large quarries to supply the industries of surrounding towns with lime for building and steelmaking, and latterly in the 20th-century cement manufacture. The Industrial Revolution also increased demand for building stone, and in the late 19th and early 20th-century, the arrival of the railways led to a large number of stone quarries being established. This industry has left its mark on the countryside, but is still a major industry: a lot of the stone is supplied as crushed stone for road building and concrete manufacture, and is moved by rail.