A variety of sedimentary structures of linear type (e.g. the ripple marks on p. 6) provide evidence of the flow direction of currents at the time of sedimentation. Measurements of the plunge or pitch of structures are often made in the field with a view to estimating palaeocurrents.

In tilted and folded strata, however, the current directions cannot be ascertained directly from the trends of such linear structures. The raw field data need to be restored to their original orientation, i.e. their orientation prior to tilting/folding. Two different situations are distinguished.

Tilted strata – fold axis is non-plunging or of unknown plunge

In these situations the usual assumption made is that the tilted strata acquired their dip by being rotated about a horizontal axis parallel to the strike (Fig. 34a).

1. 1 Plot the present orientation of the measured linear structure and the strike line of the beds (= rotation axis) on the stereogram (Fig. 34b).

2. 2 Rotate the net under the stereogram until the centre of the net's small circles coincides with the position of the plotted strike line (= rotation axis).

3. 3 Rotate the linear structure to the horizontal. This is done stereographically by moving the plotted linear structure back along the small circle on which it lies as far as the primitive circle (L → L′, see p. 64). The linear structure is now horizontal and its original trend can be read directly from the stereogram (Fig. 34b).