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Mouth bars form in river deltas with a sand-rich sediment supply and a minimal amount of wave or tidal action. In plan view, these sands form complex narrow, lenticular units that are oriented parallel to channel flow. In cross section, the mouth bars thin laterally over short distances where prodelta and delta margin silts and clays surround the sand.
Prograding sediments consist of three types of beds: bottomset, foreset, and topset. The bottomset beds lie on the basin floor or coastal shelf and have a fairly flat dip. The foreset beds are much steeper dipping and will often be concave upward toward the basin. The topset beds lie on top of the foreset beds and have a fairly flat dip (see bedding features).
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