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Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old. As, some rocks found in the Gunflint Chert region of northwest Lake Superior extend over an age of about 2.1 billion years old.