Stromatolite, a natural structure consisting of alternate layers of hardened sediment and the remains of bluegreen algae. The structure is formed by the accumulation of sand and other sediment on mats of algae in shallow waters. Stromatolites grow less than 0.02 inches (0.05 cm) a year, and over time they assume a wide range of shapes and sizes. Based on their size and growth rate, many living stromatolites are estimated to be hundreds of years old. Fossilized stromatolites are the most common fossils of Precambrian time. Some specimens dating back 3.5 billion years are among the oldest fossils known.