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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.
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.
There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.
Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures.
The only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers.
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.
These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park.
Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.
Methods for relative dating were developed when geology first emerged as a natural science in the 18th century.