The coarse surface material which protects the finer material below (Gibbins (2007) Freshwater Biol. 52, 12). The armouring of a river bed is caused by selective entrainment of smaller sediments, and reduces further entrainment of smaller sediments. An armour layer is generally well sorted, and one or two grains in thickness. Migratory during floods, it will re-form when normal flow resumes (Vericat et al. (2006) Geomorph 76, 1–2).
Subjects: Earth Sciences and Geography.