Question

Line separating different rock units on a geologic map may be

Answer 1

Line separating different rock units on a geologic map may be Fault Lines. Fault lines are made in order to release the pressure from earth's core through earthquakes. There will be fault lines everywhere in which the rocks will be weathered that causes them to change and vary from one place to another.

Answer 2

They may be Fault Lines or boundaries of Continental Plates.