Continental drift: much evidence for continental drift, but we still need mechanism to explain how and why the contents have been moving all over the group. From rock we can reconstruct how far away poles were and in which direction. Law of super position: you rock on top and position of the stack of rocks. If you plot how far away the point must be, you got a direction. We assume the physical laws to today count for the past. We had a magnetic field that"s acts constant. With pole fixed you can move the continents back and forth which was pangea at one time. Until ww2, we had no detailed picture of the ocean which covers 70% of the worlds surface. By mid 1950- there was thousands of depth sounding measurements which told us how deep the bottom was. The simple numbers provided the clues we need to solve the continental drift problem.