Dip and Strike Continued


geologic block diagram

The BLACK lines drawn on the map of A in figure 3 are the strike. Strike is parallel to the contacts. Dip is shown by a line drawn perpendicular from the strike line pointing in the direction in which the rock layer goes farther beneath the surface. At strike 2, the correct dip symbol would be the (green, red) line. At strike 3, the correct dip symbol would be the (green, red) line. At strike 4, the correct dip symbol would be the (green, red) line. At strike 5, the correct dip symbol would be the (green, red) line. At strike 6, the correct dip symbol would be the (green, red) line.

geologic block diagram

At strike 7, the correct dip symbol would be the (green, blue) line. At strike 8, the correct dip symbol would be the (green, blue) line. At strike 9, the correct dip symbol would be the (green, blue) line.

Examine how the dip changes direction around the fold. This is a plunging fold, and the plunge of the fold is (toward, away) from the viewer. An arrow should be added at the end of the axis line to point in this downward direction.

At this point, several major points must be emphasized. First, remember that strike-dip symbols are applicable only to maps. They are not needed on cross sections, because the dip is evident on those. A geologic map, however, must have them in order to show this information. Second, reexamine figure 3. Walking around this area so that you were standing on what is shown as the right side of the map would not change the geology. Standing at the opposite side, would not change the geology. The block and the map could just as easily have been drawn from any perspective around. The point here is that the loops formed by the layers in eroded plunging folds must not be confused with the vertical upfolds and downfolds. Without dip-strike symbols on the maps or other indicators, some of which will be discussed later, these loops do not indicate the geologic structure. Do not fall into this trap of confusing maps and cross sections.



Below is the finished block. Notice the added dip-strike symbol in the area of the syncline. Also, notice how the plunge at this point and the "nose" on the anticlines coincides with the plunge. These symbols are those used on a geologic map and have been put on this block diagram to help you understand the relationships. completed block


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