4. How are sedimentary rocks formed? Sedimentary rock is formed by the compression of layers of particles into a solid form. Sediments such as sand and mud settle onto the floors of oceans and lakes. Over a long period of time, several layers of sediments collect on the floor. Gradually these layers are pressed together for many thousands of years, fusing the small solid particles of mud and sand to form rock. This type of rock is called sedimentary rock.​