16.8: How Do I Draw a Strike and Dip Symbol?

How Do I Draw a Strike and Dip Symbol?

Using the quadrant system, a geologist measured the strike and dip of a rock layer as N35W, 30 NE.  How can we draw a strike and dip symbol for this measurement?

• Sketch a light grid on your paper, and label the directions, North, East, South, West (Figure 16.9).

• Using a protractor, place the small center hole or point over the place where you’ll draw the strike line, usually near the center of your grid. Lightly mark this location with a dot.
• Align the straight edge of the protractor so that it’s pointed due north, typically toward the top of the page. Since this quadrant measurement has a strike to the NW, place the protractor so that it makes the shape of a capital D. If you have a full 365° protractor, you do not need to be concerned with the overall shape. However, for both protractor types be sure 0° is towards the top of the page (Figure 16.10, left). Lightly mark the 0° point.
• Rotate the protractor 35° toward the west (to the left) until the 35° marking is aligned with true north. Be sure to read the outside numbers. Lightly mark the 35° point. The straight edge of the protractor is now aligned with a N35W strike.
• Lightly connect the marks you’ve previously made with a line along the straight edge of the protractor. To create a final strike line, darken about a centimeter or so of this line towards the center of your grid. This line is now the strike line for N35W (Figure 16.10, center).
• Now, the dip direction and measurement need to be added to complete the symbol. Determine the correct dip direction based on the measurement and the grid you’ve drawn. In this example, the dip is toward the NE, which is on the right side of the strike line. Remember, dip is perpendicular to strike. The dip line should be about 0.5 centimeter or less in length and should be perfectly straight and perpendicular from the strike line. Lastly, add the dip measurement, in this case 30°, typically this is written without a degree symbol (Figure 16.10, right).

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Using the azimuth system, a geologist measured the strike and dip of a rock layer as 330°, 30 NE.  How can we draw a strike and dip symbol for this measurement?

• Sketch a light grid on your paper, and label the directions, North, East, South, West (Figure 16.9).
• Using a full 365° protractor, place the small center hole or point over the place where you’ll draw the strike line, usually near the center of your grid. Lightly mark this location with a dot.
• Since we are now using the azimuth system, we need to measure 330° with our protractor. If you are using a half protractor, you will need to make a capital D again and subtract 330°from 365°. For both protractor types, be sure 0°/360° is towards the top of the page (Figure 16.11, left). Lightly mark the 0° point.
• Use the inside numbers to measure 330° with the full protractor. If you have the half-protractor, you must subtract 330° from 365° and then measure the resulting angle of 35° (rotate towards the west). Lightly mark this point. The straight edge of the protractor is now aligned with a 330° strike.
• Lightly connect the marks you’ve previously made with a line along the straight edge of the protractor. To create a final strike line, darken about a centimeter or so of this line towards the center of your grid. This line is now the strike line for 330° (Figure 16.11).
• Now, the dip direction and measurement need to be added to complete the symbol. Determine the correct dip direction based on the measurement and the grid you’ve drawn. In this example, the dip is toward the NE, which is on the right side of the strike line. Remember, dip is perpendicular to strike. The dip line should be about 0.5 centimeter or less in length and should be perfectly straight and perpendicular from the strike line. Lastly, add the dip measurement, in this case 30°, typically this is written without a degree symbol (Figure 16.11, right).

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