The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is a great example of Original Horizontality and the Law of Superposition, two important ideas used in 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.

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 Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative datin g' 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.

Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique.

Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate.

While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.

As he continued his job as a surveyor, he found the same patterns across England.

There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.