# Find the Outcrop Pattern of a Gently-Dipping Plane

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Recall that beds which are exactly horizontal have outcrop patterns that parallel topographic contours. We might suspect that beds which are nearly horizontal will have similar outcrop patterns, and they do, but the outcrop gradually climbs from one topographic contour to the next. It is easy to draw the outcrop patterns using topographic contours as a guide. The rule to follow is INTERPOLATE AS YOU GO. Follow topographic contours, gradually working your way from one to the next. But remember! YOU CANNOT CROSS A CONTOUR UNLESS BOTH TOPOGRAPHIC AND STRUCTURE CONTOURS INTERSECT THERE.

## Example

 1. A sill dips as shown. Its structure contours are indicated. Draw its outcrop pattern. 2. Locate the intersections of like contours. by eye. 3. In critical areas, locate a number of intermediate points by interpolation. This can be done by eye 4. Following the topographic contours, draw the outcrop pattern. See the notes below for comments on the lettered areas.

a. Here, and in a number of other places, the outcrop band leaves the map area. Extrapolate contours outside the map area to get an approximate idea how to draw the outcrop.

b. In this area, the 600-meter topographic and structure contours nearly coincide, so the outcrop pattern clearly runs along the contour.

c. We know the outcrop band must wrap around the ridge, but how far out does it extend? We see that the 600-meter topographic contour extends almost, but not quite, to the 600-meter structure contour. Thus the outcrop band does not extend to the 600-meter topographic contour, but almost.

d. Saddles are inherently ambiguous. Given that the saddle here is narrow, meaning it probably doesn't drop much below 600 meters, and it's about midway between the 600- and 500-meter structure contours, we conclude that the outcrop band probably cuts the hillside below the saddle.