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.
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.
These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.
A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The principle of intrusive relationships concerns crosscutting intrusions.
In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock.
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.