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Sedimentary rock forms in layers over time. When the layers form, plant and animal remains can create fossils in the rock. Older rock layers are usually found deeper underground. Unfortunately, movements of Earth’s tectonic plates can cause some older layers to appear higher than younger layers. This movement makes determining the age of rocks challenging. However, geologists can use fossils to help them decipher the age of rock layers. This is why fossils are so important.