From a fault map where Unit A is Cretaceous on one side and Unit C is Eocene on the other, what is the relative vertical movement of the south side?

• A. The north side is up
• B. The south side is down
• C. The south side is up
• D. The fault is strike-slip

Answer: (C)

Reading vertical movement on a fault map comes from comparing the ages of rock units on opposite sides. Here, one side shows a Cretaceous unit and the other shows an Eocene unit. Since the Eocene is younger than the Cretaceous, the side that hosts the younger rock must have moved upward along the fault relative to the other side. In other words, the south side has been uplifted (upthrown) compared with the north side.

If the south side were down, you wouldn’t expect the younger unit to appear on that side in this pattern, and a strike-slip fault would produce mainly lateral displacement with little vertical offset in the ages.