Macadam is a type of road surface composed of layers of crushed stone, typically used in the construction of roads and highways. It is made up of three layers: a base layer of large stones, a middle layer of smaller stones, and a top layer of even smaller stones. The term 'macadam' is derived from the name of the Scottish engineer John Loudon McAdam, who developed the technique in the early 19th century. TAR and ASPHALT are two common materials used in the construction of macadam roads. TAR is a sticky, black liquid made from coal, wood, or petroleum, while ASPHALT is a mixture of bitumen and aggregate.