Sedimentary rocks are the most common rocks on Earth. They form at or near the Earth's surface. Sedimentary rock is formed in layers which were laid down one by one on top of another. Some of the layers are thin, some are thick. Layers are made by deposition of sediment, organic matter, and chemical precipitates. Deposition is followed by squeezing of sediment under its own weight, and cementation. This process is called 'consolidation': it turns the sediment into a more or less hard substance.