Dating rocks and fossils and geologic events
The Grand Canyon is particularly good for this because different sedimentary rocks have different colors.
For example, the mudstone, sandstone and shale are cut by the basalt dike, so we know that the mudstone, sandstone, and shale had to be present before the intrusion of the basalt dike.
Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.
If the enclosing rock is an igneous rock, the inclusions are called xenoliths.
A more modern way of stating the same principle is that the laws of nature (as outlined by the laws of chemistry and physics) have operated in the same way since the beginning of time, and thus if we understand the physical and chemical principles by which nature operates, we can assume that nature operated the same way in the past.
Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.
Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layer will be on the bottom and the youngest layer on top.This is the same principle used to determine relative age in the trash pits discussed previously.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.