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Stromatolites are structures built by cyanobacteria (blue-green algae) in quiet waters. (more below)
An ancient example from New York
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Stromatolites in real life are mounds; during high tides or storms they become covered with sediment, then grow a new layer of bacteria on top. When stromatolites are fossilized, erosion uncovers them in a flat cross-section like this. Stromatolites are rather rare today, but at various ages in the past they were very common.

This stromatolite is part of a classic exposure of Late Cambrian-age rocks (the Hoyt Limestone) near Saratoga Springs in upstate New York, approximately 500 million years old. The locality is called Lester Park and is administered by the state museum. Just down the road is another exposure on private land, formerly an attraction called Petrified Sea Gardens. Stromatolites were first noted at this locality in 1825 and formally described by James Hall in 1847.

It may be misleading to think of stromatolites as organisms. Geologists actually refer to them as a sedimentary structure.

See a photo gallery for more pictures of stromatolites.

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