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Because the strike of a plane is a horizontal line, any line drawn between points of equal elevation on a plane defines the plane's strike. Figure 2.16a is a geologic map with two rock units, Formation M and Formation X. The contact between these units crosses several topographic contours. To find the strike of the contact, a straight line is drawn from the intersection of the contact with the 1920‐ft contour on the west side of the map to the intersection of the contact with the 1920‐ft contour on the east side of the map (Figure 2.16b). The strike of this contact is determined to be 079° (N79E), as measured directly on the geologic map using a protractor.

Using the rules of Vs, the outcrop pattern in Figure 2.16a indicates that the beds dip toward the south. To determine the exact dip, draw a line that is perpendicular to the strike line from another point of known elevation on the contact. In Figure 2.16c, a line is drawn from the strike line to a point where the contact crosses the 1680‐ft contour. The length of this line (h) and the change in elevation (v) from the strike line to this point yield the dip δ with the following equation (Figure 2.16d):

Figure 2.16 Technique for determining the orientation of a plane from its outcrop pattern. (a) Contact between Formation M and Formation X on a topographic map. (b) The line connecting points of equal elevation defines the strike. (c) A perpendicular is drawn to a point of contact at a different elevation. (d) Dip angle δ is found from tan δ = v/h.

The solution to this example is:

This method for determining attitudes from outcrop patterns can only be used if the beds are planar.

Figure 2.17 shows the Neogene (Miocene and Pliocene) units of the Bree Creek area. Straight lines have been drawn connecting points of known elevation on the bottom contact of the unit Tr. The strike, measured directly on the map with a protractor, is 344° (N16W), and the dip is 11°NE as determined by: