Two types of relative dating
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.
In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).
Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited.
However, by itself a fossil has little meaning unless it is placed within some context.Layers that cut across other layers are younger than the layers they cut through (principle of cross-cutting relationships).The principle of superposition builds on the principle of original horizontality.The age of the fossil must be determined so it can be compared to other fossil species from the same time period.