Plate 92

Gutter casts

Gutters, as the name implies, are scours with a more cylindrical and elongated shape as compared with flutes. Their surface can be made irregular, in detail, by minor grooves and asperities, as the example shows (unfortunately, in section view only).

Occurrences and causative mechanisms are similar to those of flute casts. At the center of the picture, we have here a fluvial crevasse deposit interbedded in continental fluvio-lacustrine clays. It is a single depositional event, starting with a conglomerate and ending with a laminated sandstone (needless to say, a catastrophic event).

Part of the pebbles and sand grains are made of white, alabastrino gypsum; this underscores that evaporitic minerals and crystals can be removed from the marginal areas where they precipitate, when the water levels drops because of protracted evaporation.

Messinian deposits of Tuscany, not far from Volterra, the type locality of alabastrino gypsum in Italy. Gypsum alabaster is quarried and used as ornamental stone.

Sedimentographica