Sedimentary rocks usually have clear horizontal layers, made as the sediments settled on the sea-bed. The top of each layer is called the bedding plane, and is often marked by a long crack. Bedding planes may be flat, or they may be tilted where the rock has been crumpled. Vertical cracks formed as the rock dried out are called joints. Coal is made from the squashed remains of swampy forests that once covered large areas 300 million years ago.
Conglomerate is a solid mass of round pebbles cemented together.
It was probably made from beach pebbles. Sandstone is coarse grains of quartz sand, cemented together by silica or calcite. Chalk is a shelly limestone, made from the skeletons, called coccoliths, of microscopic sea creatures that lived in warm seas 65-144 million years ago. Igneous rocks have a hard, mottled, crystalline look. Sedimentary rocks are much duller and softer, and you can often rub off the grains.