Flexi Says: Superposition refers to the position of rock layers. A lot can be learned by the position of rocks. We know the rocks on top are always younger than the rocks below. Knowing the relative age of rocks is very important to scientists. Relative age means age in comparison with other rocks. Are rocks older or younger than other rocks? The relative ages of rocks are important. They help scientists learn more about Earth’s history. New rock layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. This is the law of superposition. Rock layers extend laterally, or out to the sides. They may cover very large areas. This is especially true if they formed at the bottom of ancient seas. Seas are very large areas of water. Over time, sediment builds up on the seabed. They will be covered with the same types of material. As rocks form out of this sediment it will all be the same type. The rocks may be forced up above the water as Earth's plates move. Rivers may eventually run across this area. The river will cut into the rock and erode it away. The layers of exposed rock on either side of the river will still “match up.” Sediments were deposited in ancient seas in horizontal, or flat, layers. If sedimentary rock layers are tilted, they must have moved after they were deposited. Rock layers may have another rock cutting across them. Which rock is older? To determine this, we use the law of cross-cutting relationships. The cut rock layers are older than the rock that cuts across them.
