Figure 11.16Unusual sedimentary rocks and new, larger eukaryotes mark the rise of oxygen concentrations in the atmosphere between 2.7 billion and 2.1 billion years ago. (a) A banded iron formation. (b) These fossils of Grypania, a type of eukaryotic algae, are visible with the naked eye. (c) Red beds are made up of sandstones and shales cemented together by iron oxide minerals.