Deduce youngest rock layer from fossil dating
This layer, or "era" is often referred to as the age of mammals. Just like a cake, the bottom layer went down first, followed by the middle and the top.Since fossils progress from fish at the bottom to humans at the top, we have clear evidence that life evolved through time.
Basically, scientists have learned that rocks are stacked in layers containing fossils with the oldest fossils at the deepest layers, and the youngest, or most recent fossils, near the top. At the bottom of the timeline there are no fossils of modern animals.
The cracks, or faults, in the rock have slid the layers out of alignment.
Only when you view the entire area can you piece the original order back together.
The Law of Faunal Succession explains that fossils found in rock layers are also ordered in this way.
There are thousands upon thousands of layers in the earth's crust.
So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.