Sedimentary rocks are found covering a majority of the Earth's upper crust. Formed from the deposition of mud and sand that is buried and compressed to form solid rock, sedimentary rocks usually begin their life underwater, rising to the Earth's surface as the waters dry up.
Weathering and erosion break rocks into small pieces that are deposited in shallow seas, lakes or rivers. Over time, layers of deposits pile up. These layers and the water above generate pressure on the layers beneath them, compacting and cementing particles together. This process is what turns the sediment into rock.
Sedimentary rocks can have several unique structural elements that provide clues to how they were deposited. Most are found in horizontal layers, often recording a unique history of the Earth from the older, lower layers to the younger top layers. Minerals, in pieces of plants and other organic matter, are often trapped in the sediment, becoming embedded in the rock as it forms. Ripple marks can be embedded in the layers if the rock was formed in water. Mud cracks can also be preserved, indicating an environment that was wet before it dried up. Sometimes, imprints from raindrops, footprints and other indentations are preserved in the layers.