When Earth Scientists examine rocks in the field, they can work out a sequence of events recorded in the rocks and their age relative to others around them by applying simple geological principles.
In a vertical sequence of undisturbed sedimentary layers or lava flows, the oldest layers are at the bottom, the youngest at the top.
Layers of sediments when originally deposited are fairly horizontal. Deformed rocks (folded or tilted) are therefore a consequence of deformation that postdates deposition.
If an igneous intrusion (granite) contains fragments or inclusions of another rock, the inclusion must be older than the intrusion.
A hot intrusion of magma “bakes” a relatively cooler surrounding country rock. The rock that is baked during this process of Contact Metamorphism, must be older.
If one geological feature cuts across another, the feature that has been cut, is older.
Here all rock types are cut across by an intrusion of a dark colored igneous rock formation called dyke, thus it is the youngest feature.
An Earth Scientist sees 5 stages of geological events from the oldest event (1) to the relatively youngest event (5).
