Macadam

An illustration of the first macadamized road in the United States between Boonsboro and Hagerstown in Maryland in 1823; in the foreground, workers are breaking stones "so as not to exceed 6 ounces (170 g) in weight or to pass a two-inch (5 cm) ring".

Macadam is a type of road construction pioneered by Scottish engineer John Loudon McAdam around 1820, in which crushed stone is placed in shallow, convex layers and compacted thoroughly. A binding layer of stone dust (crushed stone from the original material) may form; it may also, after rolling, be covered with a cement or bituminous binder to keep dust and stones together. The method simplified what had been considered state-of-the-art at that point.