Plate 93

Delicate erosion marks

As in plate 90, we see here that tiny markings can fossilize very well. They can be either associated with larger forms or represent the only structures of a bedding plane. The latter case occurs more frequently in thin beds of fine sandstone and siltstone, and reflects the fact that the current had lost energy and erosional capacity. Smaller eddies thus produced smaller markings. This is not always the case, however. Delicate striations and scratches can be produced also by vigorous flows meeting small asperities of the bottom or dragging sand grains over it (see also tool marks, plates 98 to 103).

Delicate marks, seen in proper light conditions, are no less useful than flute casts in detecting the paleocurrent direction. The picture shows how clear their parallelism can be, plus the fact that they overprint larger forms of subdued and unclear morphology. Biogenic cellular structures (Palaeodictyon,  see plate 159) were here stretched and torn by the same current that made the marks, a clear evidence that the organic trace was present on the mud surface before the arrival of the current.

Marnoso-arenacea Formation, northern Apennines.