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We first find the line L12 that bisects the angle between dips 1 and 2.
One way is to construct lines perpendicular to dips 1 and 2 (in red). They intersect at C12. Bisect the angle formed by the two perpendiculars (shown in green).
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| Extend the dips out to L12. When you cross the line, continue with the dip on the opposite side. That is, use dip angle 1 on the left side of line L12 and dip angle 2 on the right. |
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| Here's another way: halfway between datum points 2 and 3, draw lines parallel to dips 2 and 3 (shown in red). Bisect the angle to find line L23. |
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Extend dips 2 and 3 to L23, then continue on the opposite side of the line with the appropriate dip. For example, extend dip 2, then where it intersects L23, continue with dip 3.
We can also extend the line that originates at datum point 1. Between L12 and L23 it has dip 2, then on the right of L23 it has dip 3.
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| Yet another approach is to average two adjacent dips. The desired line passes through the midway point between the two datum points. Draw a vertical line through that point and measure the average angle from the vertical line. Pay careful attention to dip direction so that the line has the proper slope. |
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| Lines left of L34 have dip 3, to the right they have dip 4. Continue the lines that originated at datum points 1 and 2 by extending them through L34 with the appropriate dips. |
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| L45 constructed. Lines are extended to L34 and the dip beyond is indicated. Dips 4 and 5 are very close to already existing lines and are unlikely to give any new information, so we don't need to construct lines through dips 4 and 5 for now. |
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| L56 constructed. The rocks at datum point 6 are stratigraphically lower than any we have so far seen, so we extend dip 6 to line L56, then continue to the left parallel to the previously drawn lines. |
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| L67 constructed. The rocks at datum point 7 are even lower than those at datum point 6, so we also extend dip 7 to line L67, then continue to the left parallel to the previously drawn lines. |
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| L78 constructed. The rocks at datum point 8 are still lower , so we also extend dip 8 to line L78, then continue to the left parallel to the previously drawn lines. |
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| L89 constructed. Datum point 9 lies between two already constructed lines and is not likely to tell us anything we don't already know. For now we omit drawing a line through it. |
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| L910 completed. Since point 10 falls close to an already drawn line, there is no real need to construct another line for it, either. |
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Here we have indicated the stratigraphy. It is virtually certain when you draw a cross section using strictly geometric methods that the contacts will not match exactly with their predicted positions.
What we need to do now is redraw the folds so the cross-section matches both the dips and the stratigraphy.
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Here the cross section lines are subdued.
Most of the time you can modify the fold shapes by hand to match the stratigraphy without too much trouble. Modified contacts are in black.
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