Sedimentary rock is formed by the impacting of sediments or particles into progressively tighter solids by gravity and the pressure of weight from materials above. Sediments are usually deposited by water action, like sand in a river. Over time the sand is compressed, forming a layer of materials which compresses further under the weight of additional deposits. At the point at which the compressed sand forms a true solid, it's described as a sedimentary rock. Sedimentary rocks may have other materials in their final structure, like conglomerates and breccia. Typically these rocks form as layered structures.