Stromatolites are one of the oldest bacterial microfossils known. The Peruvian Stromatolites are from the lower Proterozic period some 2.3 billion years. They are from the Eastern Andes, Bolivia, South America. Stromatolites are actually mounds of mud that have been preserved. Very few will actually contain the fossilized microbes that built them. They accumulate because blue-green algae produces a sticky film that catches mud and silt, in turn forming layers. In time theses layer build up to form mounds.