In the figure below, a series of geologic events, A-J, shows the configuration of rocks as seen from

Keep in mind a few things:

The volcanic dike (H) must be older than any strata it does not cut through and younger than any strata it does cut through, because the strata it cuts through must have been there before the intrusion of magma.

In addition, pretend the tilted strata are horizontal. That is, "D" is above "A," "C" is above "A," and so on. The Law of Original Horizontality states that strata are deposited horizontally in their original states. Tilted strata had been tilted by some geologic event after the time of deposition, but still retain their relative order.

Notice that the tilted strata are immediately overlain by horizontal strata. This can only occur if erosion has partially removed the tilted strata so they all terminate at the same depth.