Plate 64

Pebble imbrication

When a high turbulence is produced by currents or waves, pebbles can be removed and kept in temporary, or intermittent suspension.  Their weight does not allow them to travel long distances in that way; the pebbles rather make long jumps. Support is provided by fluid friction and by an arrangement similar to that of an airplane wing, i.e., dipping upcurrent. This orientation is maintained when the pebble lands and joins other companions in a shingled arrangement (imbrication ). Imbrication is commonly observed in river beds, but can be produced also by the oscillatory motion of waves on pebbly beaches. In this case, the asymmetry of orbital motion, with its predominance of landward push, make the pebbles dip seaward.

Imbrication is best displayed by platy and discoidal particles, whereby a well-imbricated gravel is rich in such elements, as the picture shows. This can be an original character, when pebbles derive from fissile rocks, but is usually acquired by hydraulic selection (shape sorting) during transport. More commonly, in fact, the original population of pebbles is a mixture of various rock types, with different sizes and shapes. The dimensions of pebbles are defined by length (a), width (b) and thickness (c), which are measured along three mutually perpendicular lines. These lines are, in their turn, orthogonal to three planes cutting the pebble along sections of maximum, intermediate, and minimum area. To get these sections, you do not need to saw the pebble: it is sufficient to project its contours on a plane from three view points at angles of 90°. Ideally, the pebble is equated to an ellipsoid, whose axes represent the three dimensions.

Platy or discoidal particles are those in which the "equatorial," or maximum area section (containing a and b axes) reaches its maximum development. The external surface intersected by c axis (normal to ab) is thus the one with maximum area, where most friction is exerted by both the fluid medium and other solids. It is the surface on which the pebble rests in its more stable position. On a smooth bottom, the pebble would lie with the plane ab horizontal; on a rough bottom, covered for example by other pebbles, the particles adhere to one another with dipping ab planes. This maximizes solid friction and resistance to being removed by a current. Imbrication is, in conclusion, a fabric of maximum stability; to dislodge imbricated pebbles, strong local eddies, sort of "hammer blows," are needed.

The pebbles seen in the picture were accumulated by waves on a beach foreshore (top of the outcrop in plate 14). Imbrication, plus size and shape sorting, make this an example of organized conglomerate.

Gravel pit in Pleistocene littoral deposits of Marche Region, Italy.

Sedimentographica