600 SEDIMENTARY PROCESSES/Depositional Sedimentary Structures ripples and are less clearly asymmetrical They are commonly straight crested with the bifurcation of crest lines The vigorous saltation leads to grains following asymmetrical trajectories: a steep uplift from the bed, and a lower angle approach and impact The impact throws other grains into motion, but the extent of this depends on the inclination of the bed at the point of impact If the bed slopes upwind, the impact is steep and energy is dissipated strongly into grains near the impact point, throwing some into saltation and advancing others by a process of creep If the bed slopes downwind, energy is dissipated more gently and grains accumulate Wind ripples reflect this behaviour and their wavelength scales to the saltation path length and, hence, to the wind strength Where wind blows dry sand across a surface that is immobile, such as a rock surface, a gravel bed, or a damp surface, wind ripples may be widely spaced and may begin to cluster together into larger forms that are incipient dunes Where the surface is damp, saltating grains may adhere on impact through surface tension and patterns of wind ‘adhesion ripples’ or ‘adhesion warts’ may form These irregular warty forms have steeper sides pointing upwind All wind ripples have quite a low preservation potential, but examples occur in the rock record Wind ripples commonly produce subparallel lamination, ‘pinstripe’ lamination with inverse grading, and rare cross-lamination It is an important component in the cross-bedding produced by aeolian dunes Vigorous wind transport commonly leads to the development of larger bedforms, ‘aeolian dunes’ (see Sedimentary Processes: Aeolian Processes), which occur in isolation on hard substrates or as parts of larger accumulations of sand Dunes vary greatly in both scale and shape, and it is quite common for dunes of several scales to occur superimposed upon one another An important distinction is between dunes with their own slipfaces and those without such surfaces Slipfaces occur downwind of crest lines where sand accumulates through grain flow avalanches Such processes commonly occur in conjunction with grainfall, whereby grains are thrown over the dune crest and accumulate on the lee side Where grainfall dominates, the inclination of the surfaces may be lower and reworking by wind ripples may occur, particularly in the lower, more gently inclined parts of the dunes (‘plinths’) Where sand supply is insufficient for the substrate to be fully covered, ‘barchan dunes’ or, more rarely, ‘linear (seif) dunes’ occur In sand seas, a variety of more complex forms occurs These include transverse dunes, barchanoid forms, and star-shaped dunes The largest forms, termed ‘draa’, commonly have superimposed smaller dunes, which leads to complex morphologies The size of the largest forms means that they change shape only over long periods of time and thus reflect a wind regime rather than a particular wind episode Some are out of phase with the current regime as a result of lag effects Internally, dune sands show complex cross-bedding at scales up to many metres thick The inclined laminae are commonly well defined and it may be possible to differentiate grainfall, grain-flow and wind-ripple laminae Wind-ripple laminae generally occur in more gently inclined intervals, whilst grain flow is commonly close to the angle of rest (ca 30 ) Aeolian dune cross-bedding is characterized by discordances or bounding surfaces at several scales Individual cross-bedded sets are separated from one another by erosion surfaces, and internally they may also be punctuated by low-angle erosion surfaces similar to the reactivation surfaces of aqueous crossbeds These record the complexity of the wind regime whereby the lee sides of dunes become sites of erosion during particular wind episodes They may also result from the migration of dunes over slipfaceless draas or the oblique migration of scour pits along the flanks of dunes Structureless Sand and Sandstone Not all sand and sandstone has visible internal lamination This happens for several reasons First, it may have been deposited in that way The dumping of sediment from suspension from, for example, a decelerating current, may have been so rapid that there was no time for the sand to be reworked into bedforms and laminae On the other hand, sand that was initially laminated may have lost its lamination through later remobilization, perhaps due to liquefaction Finally, lamination may be lost through the activities of burrowing organisms, which have the ability to totally homogenize laminated sediments (see Trace Fossils) Decelerating Flows and the Bouma Sequence Many sandstone beds in interbedded sandstone– mudstone successions show an internal vertical sequence of lamination types that are diagnostic of decelerating flows The sandstones are sharp based and may have erosional sole marks (see Sedimentary Processes: Erosional Sedimentary Structures) Above their bases, they show one or more intervals of five different lamination types that occur in a constant relative order even though they are rarely all present in the same bed These five types are: A, structureless