Plate 108

Pitted mud surfaces

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Small pits can be produced on wet mud by two distinct processes: the impact of rainfall drops (or hailstone) or the escape of bubbling gas  from the sediment. The alternative terms raindrop marks (casts),  or raindrop impact pits, and gas escape marks (casts)  can therefore be used to describe the structure. To make a distinction, the morphology of the micro craters does not help much as it is similar in both cases. Their sizes, on the other hand, should be more uniform when rain is the cause. By sectioning the sediment, one can look for evidence of internal deformation due to gas or liquid escape. The example shown here derives from the same area where mud volcanoes exist (see plate 109), and represents marks left by exploding bubbles.

Whatever the case, this relatively rare structure is to be placed together with the other indicators  of emergence, or subaerial environment. It can be preserved quite easily in dry mud.

Glossary