When snails, corals, shellfish, and marine microorganisms die, their empty exoskeletons, rich in calcium carbonate or silica, pile up at the bottom of oceans and seas. The shells break up or decay into small pieces, called bioclasts or biochemical sediments. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the shell fragments and breaking them down even further. Eventually, silica or calcite cements the individual fragments together to form a rock. The chalk you use to draw on the sidewalk comes from the shells of microorganisms that lived millions of years ago.