Sometimes, in areas of complex topography, the outcrop patterns of structures are very complex. There is no recipe for drawing these patterns. Your only recourse is to reason carefully, starting with areas where the interpretation is simple and filling in more complex areas later.
THE CARDINAL RULE: The outcrop pattern MUST be consistent with BOTH the topographic and structure contours AT ALL TIMES
1. The problem. Find the outcrop of a coal seam whose structure contours are shown.
2. Locate the intersections of like contours.
3. The outcrop can only lie between like contour intervals, shown in purple. In yellow areas the topography is higher than the coal seam, in green areas lower.
4. Start by drawing the outcrop pattern for areas where the interpretation is fairly simple. Then draw the obvious ridge and valley crossings. Finally, finish any problem areas
At the top center of the map, the 1000 meter structure contour just grazes the 1000 meter topographic contour. Obviously the outcrop crosses the ridge crest here.
Just west of the red 500-meter structure contour label, two separate purple areas join into a single one. Even though there are four contour intersections adjacent to this area, it is not hard to keep them straight. If we follow the outcrop down opposite sides of each valley, it's obvious that there have to be two stream crossings. The narrow neck of the purple area passes between the 600-meter topographic contour and the 500-meter structure contour, so clearly the coal seam must be beneath the surface in this area.
The coloring scheme used in (3) is useful in extremely complex areas to help keep track of where the outcrop can occur. It shows that the topographic and structure contours intersect to form boxes, often very irregular ones, and that the outcrop band enters at the lower corner and exits at the higher one.
Any time you see a purple band bounded on one side by green and on the other by yellow, the outcrop must pass between the contours. The reason is that the topography spans an entire contour interval and therefore must intersect the bed somewhere in that interval. This is fairly obvious when the two pairs of contours enclose a fairly neat box, but it applies equally well in all situations. It tells you, for example, that the outcrop must pass around an isolated hilltop, for example.
A very common source of confusion in areas of complex topography is the sudden appearance of new data points along structure contours. These may be due to a new ridge or valley crossing the contour, in which case the outcrop band passes through all the points, or to the appearance of an entirely separate outcrop.
The only solution is to reason carefully, work from areas of known structure outward, and use known patterns to help figure out the unknown. Be sure at all times that your predicted outcrop is consistent with both structure and topographic contours.
One common cause of this puzzle is that structure contours cross hills and valleys that were not crossed by others. Inspect the map and draw those hill and valley crossings that you can. Decide which side of the contour the outcrop lies on by careful reading; if the outcrop lies above the 1000-m structure contour, it must also lie above the 1000-m topographic contour, and so on.
The second common cause is the appearance of an entirely disconnected outcrop area. Ask yourself: can I get there from here? If you cannot connect the two outcrops without crossing a contour--and you can only cross a contour if both structure and topographic contours intersect there--then the two points are not connected. Do not connect them. In the extreme southwest corner of the map are two disconnected points. It turns out that these cannot be connected to the main outcrop band. There is a tiny area where the topography extends below the coal seam to form a small inlier. The coal seam is exposed in just a small spot in the bottom of the valley.
If you can connect the point to the outcrop band, do it. Do not leave disconnected points or parts of outcrop bands. The two points mentioned above are left to illustrate the problem; they should actually form a small loop or patch on the map.