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. So gaps are a common occurrence in many regions.Īlso, while the layers are usually deposited in a clear order, those layers are often disturbed later on by volcanoes, rivers, mountains, and shifting continents. 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. In fact, it's rare to find all three major layers on top of one another. 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. Since fossils progress from fish at the bottom to humans at the top, we have clear evidence that life evolved through time. Just like a cake, the bottom layer went down first, followed by the middle and the top. These three layers make up a sort of 3-layer cake. This layer, or "era" is often referred to as the age of mammals. The Cenozoic is the first major layer where we find modern mammal fossils like cats, dogs, monkeys and humans. This is the current layer that is still being deposited in oceans, deserts and swamps all around the earth today. On top of the Paleozoic and Mesozoic is the Cenozoic. The Mesozoic also has the first flowering plants, birds, and mammals, though few if any birds or mammals that we know of today. Of course, dinosaurs are reptiles and that's why you won't find any until after the Paleozoic which contains the first reptiles. The next layer, the Mesozoic, is often called the age of dinosaurs. Why not? The obvious explaination is flowers had not evolved yet. 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. In the Paleozoic, you find fish, amphibian, and reptile fossils (in that order), but never dinosaurs, birds, modern mammals, or even flowering plants.
These layers represent the last 500 million years of life on earth.
The most recent three layers are the Paleozoic, Mesozoic, and Cenozoic. However, scientists have grouped the layers into major groups. There are thousands upon thousands of layers in the earth's crust. The Law of Faunal Succession explains that fossils found in rock layers are also ordered in this way. Rock layers are usually ordered with the oldest layers on the bottom, and the most recent layers on top. As these sediments stack up vertically, they often harden, forming rock layers. This law of science tells us that dirt, mud, sand and other sediments are almost always deposited in horizontal layers. Two laws, or principles of geology explain why rock layers are formed in this way. We're not talking about an abstract diagram: this is the actual record of the earth's crust, recorded in rocks around the world.īut how do we know this evolutionary sequence of layers, one on top of the other, is accurate? Why is there any order at all to rock layers? As you move towards the surface, you find fish, then amphibians, then reptiles, mammals, birds, and finally modern mammals including humans. At the bottom of the timeline there are no fossils of modern animals. It's as if rock layers are a vertical timeline. 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. One evidence is rock layers-specifically, what is called the geologic column.
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.