Find the Intersection of Two Planes


This is perhaps the most common line problem in geology. There are not many purely linear structures that persist over very long distances. Fold axes and boreholes are the most common. On the other hand, there are many cases of intersecting planes: beds and faults, beds and dikes or sills, layers in a fold and joints or foliations, and so on. One very common application of this technique is locating the edge of a planar structure (bed, dike, sill) truncated by a fault.

To solve this problem, we need to recall these facts:

To solve the problem, we construct structure contours on the two intersecting planes. Where two contours of the same elevation cross, we have a point on the intersection line. Two or more such points determine the line. When we plot the line by connecting all the intersections, we automatically show the map trace and elevation points on the line.

Two Common Pitfalls

Students very often commit these two errors. WATCH OUT!

The general principle in avoiding these pitfalls is this: always be sure the intersecting contours have the same elevation.


This problems combines several simpler operations, and the individual steps for those operations are omitted. Refer to the index for information on performing other operations.

1. Given the bed and the fault shown, find their intersection

2. Plot structure contours for one plane

3. Plot the structure contours of the second plane.

4. Draw a line through the intersections of equal contours. Find trend and plunge of the line.

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Created 5 January 1999, Last Update 30 January 2012
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