Strike of quartzite bed towards S20°W

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Quartzite bed directed (dips) towards S20°W with a dip of 45°. Dip direction = S20°W, azimuth = 200°.

$$\begin{aligned} \text{Strike} &= \text{dip direction} \pm 90° \\ &= 200° - 90° = 110° \ \text{or}\ 200° + 90° = 290° \end{aligned}$$

These two azimuths (110° and 290°) are the two ends of one line, so the strike is S70°E (N70°W).

                    N
         N70W \     |
                \   |
                  \ |
  W ----------------+--------------------- E
                  / | \
                /   |   \
     dip S20W /     |     \ S70E
                    S
   ( \ = strike line N70W-S70E,  / = dip direction S20W, at 90 deg to strike )

Answer: Strike = S70°E (N70°W).