A macadam is a type of road surface made from layers of small, broken stones that are packed tightly together. The stones are carefully sized so they lock together when compressed, creating a smooth, durable road that can handle heavy traffic and bad weather.

Scottish engineer John Loudon McAdam invented this road-building technique in the early 1800s. Before macadam, most roads were just dirt paths that turned into muddy messes when it rained. McAdam's method revolutionized transportation by creating roads that stayed firm and smooth year-round, making travel faster and more reliable.

The process works like a puzzle: workers lay down layers of progressively smaller stones, then roll heavy weights over them to press everything together. The upper stones wedge into gaps between lower ones, forming a solid surface. When you walk on a gravel path that feels firm under your feet rather than loose and shifting, you're experiencing the same principle McAdam discovered.

Today, most roads use tarmac (a blend of tar and macadam that creates an even smoother surface) or asphalt, but the word macadam still appears in road construction and engineering.