macadam, form of pavement invented by John McAdam of Scotland in the 18th century. McAdam’s road cross section was composed of a compacted subgrade of crushed granite or greenstone designed to support the load, covered by a surface of light stone to absorb wear and tear and shed water to the drainage ditches. In modern macadam construction crushed stone or gravel is placed on the compacted base course and bound together with asphalt cement or hot tar. A third layer to fill the interstices is then added and rolled. Cement-sand slurry is sometimes used as the binder.