Stromatolite
(c) 2006 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Stromatolites are structures built by cyanobacteria (blue-green algae) in quiet waters. In life they 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, and just down the road is another exposure on private land, called Petrified Sea Gardens, open May to October. Stromatolites were first noted at this locality in 1825 and formally described by James Hall in 1847.
See a photo gallery from this site for more pictures of stromatolites. Another example is in the Gallery of Pet Rocks.
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
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