Relative dating exercise 1
Learn how inclusions and unconformities can tell us stories about the geologic past.We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!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.If conditions are right the remains of the dying organisms can then be preserved as fossils within the rock that formed from sediments that covered the remains.Since, all sedimentary rock is formed through the gradual accumulation of sediment at the surface over time, and since the principle of superposition tells us that newer sediment is deposited on top of older sediment, the same must also be true for fossils contained within the sediment.
It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness.
For purposes of relative dating this principle is used to identify faults and erosional features within the rock record.
Then, by applying the Principle of Cross-Cutting we are able to relatively date those processes.
Relative dating utilizes six fundamental principles to determine the relative age of a formation or event.
The first principle is the Principle of Superposition which states that in an undisturbed succession of sedimentary rock, the oldest layers are on the bottom.