When fluids like air or water flow over beds of loose sediment at speeds great enough to transport some of the grains, a great number of different kinds of bed geometries can result, depending on the characteristics of the flow and the sediment. The aim of this chapter is to summarize the present state of observational knowledge on such bed geometries or configurations, after some background on the kinds of flow patterns that are important to consider.
Why are bed configurations useful in the interpretation of depositional environments? First, they are preserved, at least in part, as primary sedimentary structures. Second, they are common, because current velocities in many important clastic depositional environments are often great enough to transport sediment over the bed. Third, they are richly varied, because they depend, albeit complexly, on both the nature of the generating flow and the size of the sediment, both of which vary substantially.
In making hydraulic interpretations from primary sedimentary structures, the underlying assumptions are that a given flow over a given sediment bed will produce a definite bed configuration, and therefore a definite stratification geometry, and that the same set of flow conditions somewhere else would produce stratification which, while different in detail, would show the same average properties and the same kinds of significant features. It is then natural to try to catalog the relationship between flow conditions and bed geometry by making observations in flumes and natural flows and then use this catalog in the reverse direction to interpret flow-generated structures present in sedimentary sequences. There are some difficulties that stand in the way of ideally complete interpretations: (1) knowledge of the basic relationships between flow and bed is incomplete; (2) relationships are in many cases hard to apply because in natural flows the bed is commonly out of equilibrium with the flow to some degree; and (3) usually the sedimentary structures are partly obliterated by the flow itself. Given present understanding and these rather severe limitations, what we can hope to do is decipher such major aspects of the physical environment as the nature and approximate strength of currents and perhaps derive additional information by looking at certain features of the structures that lend themselves to specific interpretations.