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ABSTRACT

Application of sieve and size-shape analyses (Moss, 1962) to natural and artificial bed-load deposits of shallow, unidirectional currents revealed that a textural sequence parallels one of primary structures. With increasing grain size and/or transporting power, sedimentary characteristics followed the stepwise pattern: fine ripple bed stage [RIGHTWARDS ARROW] coarse ripple bed stage [RIGHTWARDS ARROW] dune bed stage [RIGHTWARDS ARROW] rheologic bed stage.

Bed roughness (A population grain size) strongly influences sedimentation in the first three, the “bare bed” stages. Respectively, they evidently represent enclosure of the A population particles within the viscous sublayer, their protrusion above the layer which remains influential in their interstices, and the contraction of the layer to insignificance allowing almost fully turbulent flow. Transitions between these bed stages cause major changes in the A and B populations and take place on the bed. Transitions involving the rheologic bed stage, however, take place above the bed, reflecting changes in saltation intensity. The rheologic layer, a moving mass of particles kept dispersed by collisions but gravitationally held to the bed, produces distinctive sediments. Bed-load sediments can form from arbitrary mixtures in seconds of time or centimetres of travel.

Ripples and dunes form only if flow conditions near the bed are fairly steady. Ripples, evidently sensitive to temporal current direction changes, are more readily prevented from forming than are dunes. Whereas structures dominate beds between parallel flume walls, in natural flows, often rife with temporal flow perturbations, plane beds are the more common. The characteristic textural features of the bed stages are shown whether or not the structures develop.

Evidently temperature-sensitive, the fine to coarse ripple bed stage transition could probably be used as a basis for water palaeothermometry.