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.
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.
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 Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.however, this process is not enough to allow the layers to change their positions.This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.He also found that certain animals were in only certain layers and that they were in the same layers all across England.Due to that discovery, Smith was able to recognize the order that the rocks were formed.There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.