1) The document describes the different types of rock layers formed under water or by sediment deposition and their order from oldest to youngest. 2) Fossils are often found in the limestone, coal, and sandstone layers and discovering their location can help determine the age of the layers. 3) Uplifting, faulting, and erosion can disturb and mix up the original order of the rock strata, making it more difficult to determine the youngest layers.