Plate 94

Rib-and-furrow and pinch-and-swell structures

This oblique view shows a hummocky bed surface. At first sight, one is inclined to regard this morphology as the interfacial expression of real hummocky cross-bedding (see plates 47, 78) or as a rippled bed top. Actually, it is the base of an overturned sandstone bed, as indicated by the small sole markings, and the hummocks are molds of depressed portions (see inset). They reflect the morphology of current ripples, in particular of ripple troughs that partly sank in the underlying mud. The upper bed surface, not visible, is undulated, too, as indicated by the sketched section; the opposite undulations give the bed a geometry that has been named pinch-and-swell. 

The arcuate lines visible in the lower right and upper left are intersections of foreset laminae with the sandstone sole. The bisectrix of the arcs, toward the concave side, gives the paleocurrent direction (compare with plate 35). As you can check, it coincides with the direction of sole marks. The sets of arcuate lines are known as rib-and-furrow  structure.

Turbidites of the Marnoso-arenacea Formation, northern Apennines.

Photo: G. Piacentini 1970.