Otto's (1938) concept of the sedimentation unit and Apfel's (1938) definition of a phase are considered; both authors select velocity as the diagnostic parameter, and they relate the bedded structure of the unit to chance variations in velocity of the transporting current. Flume experiments on deposition of artificial units show that: A well-defined bedded structure can develop under steady-state conditions of flow (velocity) and sediment transport; velocity fluctuations represent but one of several important processes responsible for sorting and segregation of grain sizes into a bedded structure; a microchange in regimen may cause deposition of a group of laminae rather than a single lamina. The sedimentation unit must be interpreted, therefore, within wide tolerance limits.

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