The term ‘armor’ (or ‘armour’ in English usage outside the US) refers to clastic deposits in which the surface layer is coarser than the substrate. The phenomenon is widespread in gravel-bed rivers, also occurs in stony deserts (where it is usually referred to as ‘desert pavement’), and has occasionally been reported from pebbly beaches. The existence of armor is generally obvious if surface clasts are removed from a small area, and the degree of armoring can be quantified by comparing surface and subsurface grain-size distributions.
The commonest explanation for armoring is that it develops when a poorly-sorted noncohesive sediment is eroded by water or wind. The smaller clasts are more easily removed so the larger ones become concentrated on the surface as it is lowered. Thus many desert pavements are regarded as deflation lag deposits, and fluvial armor was originally recognized in degrading rivers below dams. However, as discussed below, fluvial armor can form without degradation...
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© 1978 Dowden, Hutchinson & Ross, Inc.
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Ferguson, R., Ferguson, R.I. (1978). Armor. In: Middleton, G.V., Church, M.J., Coniglio, M., Hardie, L.A., Longstaffe, F.J. (eds) Encyclopedia of Sediments and Sedimentary Rocks. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-3609-5_9
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DOI: https://doi.org/10.1007/978-1-4020-3609-5_9
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