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Three rules of relative dating

Thus, we know that the dike is younger than the mudstone, sandstone, and shale.

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Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock.This principle is often useful for distinguishing between a lava flow and a sill.Sediment deposited on steep slopes will be washed away before it is buried and lithified to become sedimentary rock, but sediment deposited in nearly horizontal layers can be buried and lithified.Thus rocks that we now see inclined or folded have been disturbed since their original deposition.In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.

Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.

Once these age relations were worked out, another principle fell into place - the principle of fossil succession.

We discuss the 7 principles of stratigraphy first and then see how these apply to fossils.

Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.

Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.

In the diagram to the right,the fault cuts the limestone and the sandstone, but does not cut the basalt.