598 SEDIMENTARY PROCESSES/Depositional Sedimentary Structures surface grow beyond a critical height, they move rapidly upstream and break, a case described as ‘antidunes’ Breaking antidunes temporarily wipe out the undulations and the cycle of growth and breaking is repeated This whole family of transport mechanisms is referred to as ‘upper flow regime’ transport Standing waves and antidunes are highly ephemeral features and preservation of their deposits is rare However, where plane bed conditions coincide with bed aggradation, distinctive parallel and horizontal lamination forms This may be accentuated by mica and gives rise to flaggy sandstones that split easily parallel to bedding The bedding surfaces themselves show a distinctive lineation (‘parting lineation’) (Figure 10) parallel to the current, which is a useful palaeocurrent indicator Rarely, parting lineation occurs on the bedding surfaces of undulatory lamination, indicating the chance preservation of standing waves Figure Standing waves on the surface of a shallow, rapidly flowing stream In phase undulations on the underlying, rapidly moving sand bed may produce undulating lamination with low preservation potential Figure 10 Upper bedding surface of parallel laminated sand stone showing a clear parting lineation or primary current linea tion This lies parallel to the direction of the shallow, rapid flow that produced it Middle Jurassic, Yorkshire Coast, England Wave Ripples and their Lamination The to-and-fro movement of water associated with waves is able to move sediment, both on its own and in conjunction with unidirectional currents Sand moved by waves is readily moulded into ripples, which are distinguished in several ways from current ripples Wave ripples, most distinctively, have straight crests, which may be rounded or quite cuspate in profile (Figure 11) Their profile is also commonly symmetrical, although not always so Where the strength of the wave surge is stronger in one direction than in the other, or where waves coexist with currents, asymmetry results Wave ripples vary quite widely in spacing, depending on the grain size, the strength of the waves, and the water depth Less common are ripples produced by coexisting wave trains with differing propagation directions These show interference patterns, reflecting the wave sets responsible As well as producing ripples in their own right, waves also modify pre-existing bedforms, particularly under emergent or near-emergent conditions Wavemodified current ripples are particularly common on tidal flats Internally, wave ripples produce small-scale crosslamination which is not always easy to distinguish from that of current ripples Interfingering bundles of laminae with opposed dips, draping laminae over symmetrical ripple crests, and discordances between internal lamination and ripple forms all suggest wave influence, but many cases are difficult to fully diagnose (Figure 12) As well as producing ripples, waves also create plane bed conditions This occurs most readily in the swash zone of beaches where the run-up and backwash of waves produce the shallow, rapid flows Figure 11 Small, straight crested wave ripples with symmet rical, rather rounded crest lines The straightness of the crest lines and the tuning fork junctions along the crests are most characteristic of wave influence Tana River Delta, Norway