Examine the rock formation for layering of different types of rock. Sedimentary rock forms by material deposits made over time, so the material on top is deposited later than the material below it. Therefore each of the layers, or strata, of rock, is older than the layers above them, unless the rock layers have been disrupted by geologic action.
Compare the rock strata in one rock formation with the exposed strata elsewhere in a region. Similarities in layers of rock imply that the two sites underwent the same series of geological events at the same time, so a rock from one layer at one site would be the same age as a rock from the matching layer at another site. Differences in the strata provide evidence of localized geological events that occurred at one site but not (or not to the same extent) at the other, such as a magma intrusion, an earthquake or excessive erosion.
Examine the rock formation for signs of geologic action other than sedimentation. Erosion may have removed one or more layers of sedimentation, in which the age gap between two adjacent layers may be greater. Magma intrusions may have cut through or between layers of other rock, in which case the igneous rock formed from the intrusion will be younger than the adjacent layers of rock. A tectonic plate shift may have have cause a series of strata to fold, warp or break, shifting the orientation of the rock layers.
Examine sedimentary rocks closely for embedded fossils, using a magnifying glass to help find and identify any small fossils. The types of fossils present will show what period the sedimentary rock was formed in. Trilobite fossils are from the Paleozoic era, while dinosaurs lived in the Mesozoic era. Therefore, rocks containing trilobite fossils will be much older than those containing dinosaur fossils. Regardless of where they are found, rocks containing the same types of fossils will have formed in roughly the same time period.