The rocks at the vast majority of exposures possess some kind of planar structure. In most sedimentary rocks a planar structure known as bedding is visible (Fig. 1a). This is a primary feature formed at the time of deposition and is a layering characterized by compositional, textural or grain-size variations. Some igneous rocks possess an equivalent structure called primary igneous layering produced by the accumulation of crystals settling out from a magma. The orientation of these primary planar structures reflects the mechanics of the deposition process, and measurements of their orientation can yield information about the palaeohorizontal, the direction of flow of currents, etc.

Foliation is a general term for all pervasively developed planar structures found in rocks. Sedimentary bedding falls under this heading as do planar structures resulting from deformational and metamorphic processes. The latter are secondary foliations and include rock cleavage and schistosity. Some foliations are defined by compositional variations; others, such as slaty cleavage, by a parallel alignment of grains or mineral aggregates.

The directions of cleavage planes are frequently measured for the purpose of estimating the directions of geological strains in rocks. Figure 1b illustrates rocks containing two foliations together: a primary foliation (bedding) and a secondary foliation (cleavage).

Gneissic banding (Fig. 1c) is a common feature of coarse-grained metamorphic rocks such as gneisses. This secondary foliation is a compositional layering defined by the concentration of particular minerals.