Définition : Gravel is an environmental material which is composed of pieces of rock that are at least two millimeters (2mm) in its largest dimension and no more than 75 millimeters.
Définition : A piece of rock with a grain size above 300 millimetres in diameter. Smaller boulders are, at times, referred to as rocks or stones. [ https://en.wikipedia.org/wiki/Boulder]
