590 SEDIMENTARY PROCESSES/Erosional Sedimentary Structures Figure Flute casts on the base of a turbidite sandstone bed showing the noses pointing in the upstream direction Eocene, San Sebastian, Spain of the flute, the axis of the separation eddy would have been transverse to flow and the axes would have rotated parallel to flow along the downstream flaring margins ‘Longitudinal furrows’ occur on the bases of sandstone beds as patterns of broadly parallel ridges that have a rounded profile and are separated by narrower grooves This reflects an erosional morphology of rounded parallel furrows separated by cuspate ridges Along the furrows, it is quite common to find rounded noses that all point in the same direction, similar to small flutes Indeed, there is a continuous series of erosional forms between clear flutes at one end and smooth parallel furrows at the other The furrows are thought to result from the erosive effects of paired spiral eddies with their axes parallel to flow and with opposed senses of rotation The furrows correspond to divergence zones where eddies direct flow down on to the bed, whilst the cuspate ridges lie beneath the convergence zones where flow components lift off from the bed ‘Gutter casts’ are features that seem to occur preferentially in shallow marine settings where storm currents are typically the agent responsible They differ from longitudinal furrows in being larger and separated from one another by areas of flat bedding surface rather than cuspate ridges (Figure 6) They are typically many centimetres wide and a few centimetres deep and they may be separated by flat sectors up to metres in width Their fills sometimes carry a coarse lag in the base, often made up of fossil fragments In plan view, they tend to be rather continuous, may be somewhat sinuous, and may split and rejoin, suggesting an anastomosing pattern They probably result from rather sustained currents, possibly driven by wind or storm effects All the scours related to fluid scour have a strong orientation parallel to the current, which can be used Figure Lower bedding surface of a limestone bed from within a shallow marine interbedded limestone mudstone succession showing gutter casts Ordovician, north west Greenland to infer palaeoflow direction With longitudinal scours without clear noses, it may be impossible to infer the sense of movement Tool Marks These constitute a diverse suite of structures that are produced by the interaction of objects (‘tools’) carried by the flow with a soft cohesive substrate The shape and scale of such structures vary with the nature of the tools and the way in which they impinge on the bed (Figure 7) Tools marks can generally be distinguished from structures produced by fluid scour by the fact that they tend to have sharp, often angular relief, reflecting the shape of the tool, compared with the much smoother shapes of fluid scour marks The tools themselves are seldom seen associated with the tool marks that they created, and one can only imagine or infer what they may have been They are most likely to have been pebbles, mud rip-up clasts, fossils, or wood fragments In some cases, delicate relief on the marks can be related to features of the tool, such as ribbing on a shell Tools may interact with the bed in a variety of ways, and a loose terminology is applied to the results Where tools remain in contact with the bed for some distance, they gouge ‘grooves’, usually with sharp margins and sometimes changing in form as the tool rotates slowly Where the tools are in intermittent contact with the bed, a string of similar impact marks