Plate 101

Marks produced by dragging objects: chevron cast

Chevron, or skim casts are made by objects that graze the bottom but do not sink into it (in anthropomorphic terms, they tickle the mud). The response is an oscillatory deformation similar to the waves produced by a hull on a water surface. The resulting V-shaped wrinkles are ideal indicators  of the paleocurrent direction (they point down current).

The example shown here, from the Paleozoic Aberystwyth Grits of Wales (U.K.), represents the local transformation of a groove. The grooving object, coming from the right, was raised a little, produced the chevron mark, then sank again reforming and extending the groove.

The rest of the bed surface is fairly smooth, exception made for scattered marks of squat shape, deriving from the momentary impact of bouncing and skipping objects.