Three principles of sedimentary rocks are critically important for interpreting geologic history.
The first principle is that sedimentary rocks always form on the surface- one atop the other. They are deposited by settling of sand, silt, clay, gravel, and mud out of water; or by chemical precipitation of minerals formerly dissolved in water. Thus, when the Hermit Shale formed on Earth's surface, the Coconino Sandstone, which lies above the Hermit Shale, did not yet exist.
We call this the principle of superposition, In an undeformed stack of sedimentary rocks, the oldest is on the bottom and the youngest is on top.