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Numerical dating determines the actual ages of rocks through the study of radioactive decay.Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.Geologists use this type of method all the time to establish relative ages of rocks.Nicolaus Steno was a 17th-century Danish geologist.The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence.Man-made intrusions and activity in the archaeological record need not form chronologically from top to bottom or be deformed from the horizontal as natural strata are by equivalent processes.Some archaeological strata (often termed as contexts or layers) are created by undercutting previous strata.
Geologists establish the age of rocks in two ways: numerical dating and relative dating.An example would be that the silt back-fill of an underground drain would form some time after the ground immediately above it.Other examples of non vertical superposition would be modifications to standing structures such as the creation of new doors and windows in a wall.It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks. Say you have a layer of mud accumulating at the bottom of a lake. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer.Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. The forest layer is younger than the mud layer, right? When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history.
Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.