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.
Prior to the late 17th century, geologic time was thought to be the same as historical time.
Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.
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.
Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.
Thus, we can deduce that the mudstone and shale are older than the rhyolite dike.
But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.
Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.
Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.