In outcrop-scale folds it is often possible to measure the orientations of geometrical features such as the fold hinge line and fold limbs directly with a compass/clinometer in the field. In these instances the stereographic projection is used to manipulate these data, including rotation (see pp. 64–9) or calculation of the interlimb angle (pp. 46–7). For folds which are larger than outcrop-scale, the stereographic projection can be employed as well for the estimation of the orientation of the fold axis and axial plane.

Are the folds cylindrical?

The fold axis is defined with respect to folds which are cylindrical. Cylindrically folded surfaces (Fig. 22a, 22b) are surfaces with the form that could be swept out by a straight-line generator moving in space but remaining parallel to itself. Cylindrical folds have the property that their shape remains constant in serial sections. The fold axis is the orientation of the generator line of such surfaces. It is parallel to the hinge lines (the lines of sharpest curvature) of individual folds in the surface.

A cylindrical fold can be easily recognized from the measurements of orientations of the folded surfaces (e.g. bedding) taken at a variety of locations across the fold (Fig. 22d). The normals to the bedding planes in a cylindrical fold are all parallel to a single plane, the profile plane. Therefore, to check whether a fold is cylindrical or not:

1. 1 Plot on a single stereogram all the readings of the bedding (or other surfaces which have been folded) as poles (Fig. 22e, 22f).

2. […]