How to do relative dating
Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault.Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.The principle of faunal succession is based on the appearance of fossils in sedimentary rocks.As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found.This is because it is not possible for a younger layer to slip beneath a layer previously deposited.The only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers.
Photo from Glen Canyon National Recreation Area, Utah.
A similar situation with igneous rocks occurs when xenoliths are found.
These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.
Explanations: A – folded rock strata cut by a thrust fault; B – large intrusion (cutting through A); C – erosional angular unconformity (cutting off A & B) on which rock strata were deposited; D – volcanic dyke (cutting through A, B & C); E – even younger rock strata (overlying C & D); F – normal fault (cutting through A, B, C & E).
The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.
Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.