Rock layer dating
Rock layers are usually ordered with the oldest layers on the bottom, and the most recent layers on top.
The Law of Faunal Succession explains that fossils found in rock layers are also ordered in this way.
This law of science tells us that dirt, mud, sand and other sediments are almost always deposited in horizontal layers.
As these sediments stack up vertically, they often harden, forming rock layers.
The Cenozoic is the first major layer where we find modern mammal fossils like cats, dogs, monkeys and humans.
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.
These layers represent the last 500 million years of life on earth.In the Paleozoic, you find fish, amphibian, and reptile fossils (in that order), but never dinosaurs, birds, modern mammals, or even flowering plants.Think of that: despite the billions of plant fossils in the Paleozoic layer, nobody has ever found one fossil of a flower, including any kind of deciduous tree or even a single blade of grass. The obvious explaination is flowers had not evolved yet. While everybody understands that black bears are related to grizzly bears and we can even figure they are related to extinct bears, lots of people wonder how scientists can be so sure that bears are related to salmon as well.One evidence is rock layersspecifically, what is called the geologic column.In fact, it's rare to find all three major layers on top of one another. Well the first obvious answer is that even in the world today there are places where sediments (layers) are deposited but in other places (like mountains) they are eroded. Also, while the layers are usually deposited in a clear order, those layers are often disturbed later on by volcanoes, rivers, mountains, and shifting continents. If you were to stand on the cliff to the left side of the cross section, you would see the top layer in two places.The cracks, or faults, in the rock have slid the layers out of alignment.These plates "float" on the hot, soft mantle below the crust.We can actually measure the movement of the plates using satellites in space.Since fossils progress from fish at the bottom to humans at the top, we have clear evidence that life evolved through time.Of course, there isn't one place in the world to go and see every fossil animal from all time stacked one on top of the other.