There are two ways of describing the orientation of a geological line:

Plunge and plunge direction In this system the orientation of the line is described with reference to an imaginary vertical plane which passes through the line (Fig. 4a). The angle of tilt of the line measured in this vertical plane is called the angle of plunge. The plunge is measured with a clinometer which is held upright and with the edge of the instrument aligned with the linear structure (Fig. 4c). The plunge direction is parallel to the strike of the imaginary vertical plane passing through the plunging line (Fig. 4a). It is measured by placing the edge of the lid of the compass along the lineation and, with the plate of the compass held horizontal, measuring the compass direction of the down-plunge direction (Fig. 4d).

These measurements are written as

angle of plunge–plunge direction.

For example, a linear structure with orientation 30–068 is tilted down or ‘plunges’ at an angle of 30° towards bearing (compass direction) 068° (Fig. 4a). Line 0–124 could equally be written 0–304 because a horizontal line can be said to plunge in either of two directions 180° apart. A linear structure with a plunge of 90° is vertical and its direction of plunge is not defined.

Pitch Many linear structures are developed on planar structures; for example, slickenside lineations are to be found on fault planes.