Locate the Intersection of a Plane with a Ridge or Valley

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

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As a general rule, outcrop patterns make "V" shapes that point downdip where they cross valleys and updip where they cross ridges. Think of the "V" across a valley as a dip symbol on a map.

1. Locate the outcrop of this marker bed given the three exposures indicated. 2. Construct structure contours on the bed. 3. Locate the intersections of like contours and locate the intersection of the bed with the hillsides 4. Sketch the valley and ridge crossings. Interpolate to find the most likely location.

Exceptions occur when the slope of the topography is steeper than the structure.

1. Locate the outcrop of this marker bed. 2. Construct structure contours on the bed. 3. Locate the intersections of like contours and locate the intersection of the bed with the hillsides. Note that there is no way for the outcrop band to exit the 700-800 meter elevation interval shown in purple. 4.Thus, the only possible conclusion is that the outcrop band closes around the end of the ridge, making a V that points down the ridge.

The outcrop band cannot exit the area shown in purple in (3.) because it cannot exit without crossing both an elevation and a structure contour, but the 700-meter structure contour does not pass through the area shown. Thus the outcrop band has to close around the end of the ridge. It does so because the bed dips more shallowly than the slope of the ridge crest.

Rather than trying to memorize rules about when outcrop patterns close in which direction, it's better to have a firm grasp of three-dimensional visualization. Outcrop patterns can get much more complex than this.

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Created 18 March 1999, Last Update