Measurement of the attitude of folded bedding or other foliation, if analysed stereographically, can permit certain geometrical properties of the folds to be determined. These deductions can be made even in areas where rock exposures are scarce, and as a result folds are hardly ever seen. An example of the type of technique employed was described on p. 44 where, if the poles to bedding plot along a great circle, this is interpreted to mean that the folds involved are cylindrical.

In addition, the way in which the poles are distributed within the great circle girdle can suggest other features of the folding to be discussed below, e.g. tightness (inter-limb angle), curvature and asymmetry. Unfortunately there exist other factors which potentially influence the spread of poles within the great circle, in particular the distribution of the sites at which measurements of the bedding have been made. The pattern on the stereogram can be biased by this sampling effect. For this reason care must be taken with the deduction of fold shape from stereograms.

Fold tightness

The range of orientations of the folded surface is restricted in an open fold (e.g. Fig. 24c) but is greater in a tight fold (Fig. 24i). The stereograms resulting from open structures show a lower degree of spread of poles than for tight folds. In Figure 24b, 24c and 24f where fold profile shapes are illustrated together with representative stereograms, a pole-free part of the great circle can be identified.