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plane bed (flat bed) A near-horizontal surface of sand or gravel. Two types of plane bed are found. Upper-stage plane beds are produced by the intense transport of sediment by high-velocity, shallow flows (upper-flow-regime conditions), and characterized by primary current lineation on the sediment surface. Lower-stage plane beds are produced only in coarse sands and gravels by flow conditions broadly similar to those which generate current ripples in finer sand. The lower-stage plane bed exhibits a series of shallow scours on the sediment surface. The accumulation of plane-bedded sediment gives rise to an internal sedimentary structure of horizontal lamination.