Introduction to structure contours

In Geosciences structure contours - as the name suggests - are curves that connect points of equal height above a datum level that are contained within a structure (bedding, unconformity, fold, fault...). Structure contours and topographic contours are similar, in the sense that they are curves joining points of equal height above a datum level.
The block diagram below illustrates structure contours of a perfectly planar surface (blue). Notice that the structure contours are (i) straight lines, (ii) parallel to each other and (iii) equally spaced. These three features are the fundamental characteristics of structure contours of a planar structure!



Geological structures are not always planar. A very common family of structures are folds. The diagram below shows a folded layer and its structure contours. The structure contours exhibit features that are strikingly different compared to the diagram for a planar layer above. You will notice that the structure contours are parallel to each other (though this is an exception rather than a rule for folded layers!), which is typical for non-plunging, cylindrical folds. Another feature of the structure contours is that (i) their spacing varies and (ii) their elevations progressively change in ascending to descending order or vice versa (i.e. from the front to back in this diagram they progress from 10m, 0m, 10m, 20m, 30m, 20m). This systematic upward and downward change in the elevations of structure contours is the main characteristic of folded layers!

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