# Find a Fold Axis From the Intersection of Bedding and Foliation

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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 If we look at the axis of each individual layer in a fold, we get a series of lines in space that define a surface, the Axial Surface. If this surface is planar, as it often is approximately, we call it the Axial Plane.
 The folding process often imparts a sheetlike grain, or foliation, to the rocks. The foliation is very often parallel to the axial plane of the fold. Compression on a large scale produces the fold, and compression on a small scale produces the foliation.
 If the fold is cylindrical, the bedding at any point on the fold is parallel to the fold axis, and the foliation is parallel to the axial plane and therefore parallel to the fold axis. Then the intersection of bedding and foliation defines the fold axis.

 The most common exception to axial-plane foliation is for the foliation to fan. However, fanning foliation usually is still parallel to the fold axis so the method of finding the fold axis from the intersection of bedding and foliation can still be applied.

## Example

 1. The problem. Given the strike and dip of bedding and foliation, what is the probable orientation of the fold axis? 2. Plot structure contours on bedding and foliation planes. 3. Determine the intersection of the two planes. 4. Find the trend and plunge of the intersection. This is the probable orientation of the fold axis.