Between 440-510 million years ago (the Paleozoic period) a warm tropical sea flooded the region, which was then near the equator. The beds of limestone that you see along the riverbank today were deposited at this time, in large part due to the life processes of organisms. Algae and bacteria were responsible for the growth of stromatolites, and the elaborate shells of lime-precipating animals such as orthocones can be seen in several places along both shores of the river today. Their principal locations and characteristics are described below.
Stromatolites in the Ottawa River are present just downriver from Westboro Beach, but the section of river near the Champlain Bridge has one of the best known displays of fossil stromatolites in the country – and certainly in any urban area. At low water periods at the end of dry summers and early fall large slabs of sedimentary rock appear along the river to expose the stromatolites and other ancient creatures. Stromatolites are biofilm layers built up by cyanobacteria. These layers encompass the most primitive life forms and are generally regarded as falling somewhere between the plant and animal kingdoms (bacteria, blue-green algae, etc.). They are regarded as biogenic structures as they are not true body fossils. Like orthocones, stromatoporoids, brachiopods, corals, etc., they thrive in warm aquatic environments and build reefs. Cyanobacteria and algae were likely responsible for the creation of the oxygen in the Earth’s atmosphere. They were the dominant lifeform on earth for over 2 billion years. Today they live in abundance in only a few localities worldwide. One of these places is Australia where a very salty protected environment has enabled them to live in an intertidal zone.
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