About 75% of the rocks exposed on the Earth's surface are sedimentary rocks. Sedimentary rocks are also products transformed by geological processes, but because they form on or near the surface, they are very different from metamorphic rocks.  

Wind, flowing water, and glaciers continuously weather and erode the original rocks, breaking them into loose materials of varying sizes, which are collectively called sediments. Gravel, sand, silt, and clay are common types of sediments classified by particle size. When water flows down a slope, it carries sediment particles into lakes and oceans, where they settle. As loose sediments continue to accumulate, their particles are eventually compacted and cemented together, forming solid rock. Sandstone, conglomerate, and mudstone are sedimentary rocks formed in this way.  

Sedimentary rocks are formed by the accumulation of sediments. They have three basic types:  

1) Clastic sedimentary rocks (formed from weathered fragments), such as breccia, conglomerate, sandstone, and shale;  
2) Chemical sedimentary rocks (formed by precipitation of dissolved substances from solution), such as rock salt and certain limestones;  
3) Organic sedimentary rocks (formed by the accumulation of plant or animal remains), such as coal and certain limestones.  

Sandstone is a type of clastic sedimentary rock, mainly composed of weathered fragments of sand size (diameter 1/16 to 2 millimeters). Situations where a large amount of sand accumulates include beaches, deserts, floodplains, and deltas.

Conglomerate is a clastic sedimentary rock mainly composed of rounded gravel particles of gravel size (diameter greater than 2 millimeters). The spaces between the pebbles are usually filled with smaller particles and/or a chemical cement that binds the rock together.

Limestone is a rock mainly composed of calcium carbonate. It can be organically formed from accumulated shells, coral, and algal remains, or it can be chemically formed from calcium carbonate precipitates in lake or seawater. Limestone has a variety of uses, among which the most common are producing cement, crushed stone, and acid neutralizers.

Limestone is a rock mainly composed of calcium carbonate. It can be organically formed from accumulated shells, coral, and algal remains, or it can be chemically formed from calcium carbonate precipitates in lake or seawater. Limestone has a variety of uses, among which the most common are producing cement, crushed stone, and acid neutralizers.