Plate 83

Comet-shaped obstacle marks

---

Swash and backwash, especially the last, produce these long and shallow scours behind flat pebbles lying on the sand of beach face. The water cover is very thin and does not form moatlike scours around these minimally protruding objects.

The prevalence of platy pebbles is an effect of shape sorting by waves (compare with plate 64). More globular pebbles (spheroidal and "triaxial" shapes) were certainly present in the original gravel carried to the sea by a river. Being less adherent to the bed, they were more easily removed from the foreshore and carried under water, also because gravity helped the backwash.

Glossary