16.7: Strike and Dip

Strike and Dip

Recall that the symbol for strike and dip is given along the top of the block, as shown in Figure 16.6. Note that the dip symbol (shorter line) is perpendicular to the strike symbol; dip degree (number) will vary depending on measurements. For more information on strike and dip, read section 15.4.

Left, a Google Earth image of tilted sedimentary rocks. Right, a strike and dip symbol overlain on a compass rose.

Figure 16.6: Right, a strike and dip symbol on tilted sedimentary beds. Left, the strike and dip symbol from the diagram on at compass rose to illustrate directional and use of the symbol. (CC-BY 4.0, Chloe Branciforte via Google Earth)

Strike and Dip Measurement

Geologists typically use a Brunton compass Links to an external site. to measure strike and dip (Figure 16.7). Strike is measured along the plane by using the leveling bubble on the compass. Dip is measured by laying the side of the compass perpendicular to the strike measurement and rotating the horizontal level until the bubble is stable and the reading has been made.

A Brunton Compass, Far left, the mirror, with line and sighter on top of the mirror, and window to look through the window. Middle region, the face, with azimuth degrees around the outside of the face measuring 0-359 degrees, the needle; the bubble level; the inclinometer (set at ~30 degrees), a magnetic declination setting (green point on the outer ring, set to ~15 degrees). Exterior to the face is the damping button and a sighting arm.

Figure 16.7: A Brunton Pocket Transit, a specialized compass used by geologists and other field-based surveying over long distances. (CC-BY-SA 3.0; Matt Affolter (QFL247) Links to an external site.; modified by Chloe Branciforte)

Recording strike and dip can be done using two different systems, the Azimuth System or the Quadrant System (Figure 16.8), and the choice between the two is dependent on the geologist, location, or organization for which the data is collected or produced. A strike can be given as a quadrant compass bearing of the strike line (N25E for example) or using a single three-digit number representing the azimuth, where the lower number is usually given. In our example of N25E, the azimuth reading would be 025° (see Table 16.2 for other examples.) Some students will find they prefer one system over the other. This is typically a personal decision; however, many geologists (and geology majors) find it important to know both systems, as they may be asked to use both over the course of their career.

Left, azimuth compass bearings. Right, quadrant compass bearings.

Figure 16.8: Left, azimuth compass bearings. Right, quadrant compass bearings. (CC-BY 4.0, Chloe Branciforte, own work)

Table 16.3: Compass systems and their relationship to strike and dip. (CC-BY 4.0, Chloe Branciforte)
System Strike & Dip Description
Quadrant N50W, 78NE -
Quadrant N70E, 30SE -
Quadrant N90W, 33N Strike direction, due west. Dip direction, due north.
Quadrant N70W, 90 Vertical dips have no quadrant direction.
Azimuth 315, 78NE -
Azimuth 70, 30S -
Azimuth 270, 33N -
Azimuth 290, 90 -