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 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.
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".
Laying Telford paving in Aspinwall, Pennsylvania in 1908
John Loudon McAdam
A macadam road in Nicolaus, California, c. 1850s
John Loudon McAdam was a Scottish civil engineer and road-builder. He invented a new process, "macadamisation", for building roads with a smooth hard surface, using controlled materials of mixed particle size and predetermined structure, that would be more durable and less muddy than soil-based tracks.
John Loudon McAdam, 1830, National Gallery, London
Memorial stone to John Loudon McAdam