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As a result, xenoliths are older than the rock which contains them.The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.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.Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.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 states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.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.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.

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