Plate 23

Tractive laminae

Sedimentary structures can be seen not only in the field but also in buildings and monuments, where natural stone blocks were employed. Various types of sandstone, for example, can be found in friezes, decorations, window-sills. Sometimes, sandstone is used for load-bearing elements, such as the columns in this picture, which are made of New Red Sandstone, a continental formation of Permo-Triassic age cropping out in the U.K. and Ireland.

In many cases the stone is chosen simply on the basis of local availability and economic convenience (cost of transport, labor, etc.). It is possible, however, to suppose that there was some aesthetic reason, too. One could be, for instance, the effects of light on the fine relief of the sandstone, as in the case illustrated here, where differential cementation and weathering emphasize lamination. Laminae in sand and sandstone are the most typical expression of tractive mechanisms, which are active in the bed load of a fluid-driven current. The bed load is formed by particles that move close to the bottom (see inset); due to their high concentration, the grains collide frequently.

The effects of the collisions, which increase with the momentum (current speed and particle mass), are twofold: sorting and abrasion. Like billiard balls, the grains move away after each impact and tend to join particles of similar characteristics (weight and size). The net result of this "similar seeking similar" mechanism is to sort the grains and assemble them, during the movement, into relatively homogeneous arrays, which constitute the cores of laminae. At the beginning of traction, a few grains are mobilized but, with increasing current velocity, the whole bed becomes mobile and forms a traction carpet.  The solid friction between moving grains and between them and the stationary bottom determines a resistance to transport; when this resistance exceeds the fluid stress, the grains stop, one string after the other, and a lamina is deposited.

A fresh supply of grains will accumulate new laminae on top of the previous ones. If the sand is deposited too rapidly, virtually in mass, the current has no time to sort the grains and form the laminae. Or, a faint lamination is only produced, and the water escaping upwards from the compacting sand deforms the laminae (as happened in the block on the upper right side of the picture).

Grain collisions also cause mechanical wear, manifested by the detachment of tiny chips that leads to rounding of corners and edges. Attrition is more intense in larger clasts, whereby pebbles are more rounded than accompanying sand for the same distance of travel.

Photo: V. Rossi 1992.