Plate 90

Flute and crescent casts

A variety of flute morphology is shown by this slab of sandstone. The preservation of morphological details by the molding sediment can also be appreciated.

The parallelism of flute casts is impressive. Two oblique traces are visible, but were caused by different mechanisms. One is to the left of the pencil, and is a tool mark (a scratch made by an impacting or dragged object), the other (near bottom center) was left by a burrowing organism. The burrow is cut into segments by flutes, which implies that it was preexistent to scouring.

Scattered among the flute casts, are small, elongated crescent casts (see plates 81-83), suggesting that tiny obstacles protruded from the bottom. Some of them could have been brought by the eroding current itself. Nowhere was the bottom perfectly smooth; numerous small marks studded it along with the flutes.

Marnoso-arenacea Formation, northern Apennines.