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A stromatolite is a preserved structure in sedimentary rock that is a series of thin layers of sediment. These layers formed when a colony of algae trapped loose sediment particles.

Stromatolites occur in rocks that range in age from very recent to more than 3.5 billion years old. Ancient stromatolites are the oldest evidence of life that is visible without a microscope. They are also the most common evidence of life found in rocks dating from about 3.5 billion years ago to 1.5 billion years ago. The algae that form them are of the blue-green variety. Also called cyanobacteria due to their primitive nature, blue-green algae carry on photosynthesis to produce their own food. Thin algal mats (colonies) grow today in almost any moist environment—rocky stream beds, boat piers, shower stalls, etc.

Stromatolites are not true fossils. They contain no preserved body parts (bones, for example) or impressions (for example, footprints). Stromatolites consist of only layered sediment. They form when algal mats trap sediment on a sticky "carpet" of thin, thread-like filaments. Currents or waves move sediment over algal mats located in shallow water. Fine sediment—especially particles less than 0.0025 in (1/16 mm)—sticks to the filaments. Sediment collects on the algal filaments like dirt sticking to a rug where you spilled a soft drink. Eventually the sediment limits the amount of light available to the algae for photosynthesis. In response, the Stromatolites at Hamelin Pool in western Australia. Photograph by Francois Gohier. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. algae send new filaments up through the accumulated sediment layer, creating a "clean carpet," that is, a new algal mat. Another sediment layer then begins to accumulate. In this way, many thin layers of sediment form. Later, the algal filaments usually decay so that only the sediment layers remain.

Most stromatolites occur in limestone, rather than in sandstone or shale. The sediments that form limestone are usually bound together (cemented) soon after the particles accumulate. Cementation occurs much later in most sandstone and shale. Thanks to this early cementation, stromatolites in limestone are more likely to avoid destruction by burrowing animals and other disruptive processes. Stromatolites also are less common in rocks younger than about 500 million years old. Plant-eating animals (herbivores) probably began to evolve about 850-900 million years ago. For the next few hundred million years, herbivores became increasingly common, and their grazing activities began to destroy more and more stromatolites. By 500 million years ago, preservation of stromatolites became much less likely.

Ancient stromatolites probably populated the sea bottom in much the same way as modern coral reefs, preferring shallow, well-lit areas. However, modern stromatolites are common only in very salty, shallow ocean waters or in highly mineralized lake water. These high salinity (salty) environments are hostile to many organisms, but stromatolites thrive there because of the absence of grazing herbivores. In this setting, algal mats build dome- and column-shaped stromatolites that reach as much as 3 ft (1 m) in height.

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