The types of sedimentary rock are based mainly on clast size but also on amount of layering. Re clast size it goes from conglomerate with up to cobble clasts, to sandstone of coarse, medium and fine clasts and siltstone with finer still clasts and down to a shale with very fine clay sized particles. Sandstone, siltsstone and shale can have quite thin layers though the thinnest in what is called a fissile shale tends to be in shale with perhaps a few mm layer thicknesses. Chemical composition can also pay a role such as a limestone made up of tiny fragments of shells sometimes called a coquina and also marl.