SEDIMENTARY PROCESSES/Depositional Sedimentary Structures 599 Figure 12 A wave ripple bedding surface with clear, symmet rical ripple profiles, underlain by a unit of ripple laminated sand showing bundled laminae and trough forms in a view normal to the crest lines Namurian, County Clare, Ireland Figure 13 Sandstone bed with undulating lamination with convex upwards sectors and undulatory erosion surfaces This assemblage of hummocky and swaley cross stratification is typical of deposition from storms under conditions of coexisting, intense current and wave activity Upper Jurassic, Dorset, England needed Parallel lamination and parting lineations result if the sand is preserved Wave–Current Interaction During storms, shallow-water shelf or nearshore areas are commonly subjected to energy regimes involving a combination of waves and currents Under such conditions, sand falling from suspension may be reworked into low-relief, rounded forms and may be re-eroded by higher energy surges The result is an undulatory lamination, characterized by gentle convex-upwards patterns and gentle scours Where the convex-upwards laminae dominate, the structure is called ‘hummocky cross-stratification’, and where the scours dominate, it is termed ‘swaley cross-stratification’ (Figure 13) Heterolithic Lamination In settings in which energy levels fluctuate and in which there is a significant amount of sediment in Figure 14 Heterolithic interlaminated sand and mud typical of many tidal settings where energy levels fluctuate between strong currents capable of transporting sand and still conditions where mud falls from suspension to drape the bed morphology Lower Cretaceous, Isle of Wight, England suspension, it is common for mixed sand–mud sediments to be deposited These record the alternation of suspended and bedload sediment deposition and are a particular, although not exclusive, feature of tidal settings The sandy components are often ripple laminated with ripple profiles commonly preserved, so that current and wave forms can be distinguished Collectively, these interlaminated sediments are termed ‘heterolithic’ (Figure 14) and are subdivided using the proportions of sandy and muddy components Where mud dominates and sand forms isolated ripple lenses, the term ‘lenticular bedding’ is applied, whilst ripple-laminated sand with discrete mud drapes is called ‘flaser bedding’ More or less equal components are referred to as ‘wavy bedding’ Some confusion surrounds the application of these terms Laminae rich in mica and carbonaceous debris in normal ripple cross-laminated sand can look superficially like flaser bedding, but the term is best reserved for cases with a clear separation of sand and mud Aeolian Bedforms and Internal Bedding Wind, blowing over dry sand, initiates movement in much the same way as water Above a certain critical velocity or shear stress, grains begin to move as bedload However, in comparison with water, wind moves a much narrower range of grain sizes in this way The buoyant effect of water is much greater than that of air and, in addition, the viscosity of water dampens collision impacts between grains in saltation The result is that sands moved by air tend to be very well sorted and also well rounded The first bedforms to develop on dry sand are ripples These have low relief compared with aqueous current