A clastic sedimentary rock comprising an aggregate of sand-sized (0.06–2.0-mm) fragments of minerals, rocks, or fossils held together by a mineral cement. Sandstone forms when sand is buried under successive layers of sediment. During burial the sand is compacted, and a binding agent such as quartz, calcite, or iron oxide is precipitated from ground water which moves through passageways between grains. Sandstones grade upward in grain size into conglomerates and breccias; they grade downward in size into siltstones and shales. When the proportion of fossil fragments or carbonate grains is greater than 50%, sandstones grade into clastic limestones.