Stromatolites (Brecciated Basal)
Lower Ordovician (495 million years old)
Oneota Formation, Buffalo County, Wisconsin

It derives its name, Brecciated Basal, from the fact that it originates at the base (Basal) of a large overlaying stromatolite structure. Brecciated (angularly broken) signified that, in fact, the original stromatolitic structures were shattered into irregular shapes, probably in a surf, and were then buried under the stromatolite colonies that subsequently grew above. Mineral replacement has been hard at work with a bluish gray silica predominating, making for an attractive creation through the interaction of biology and chemistry. The mineral replacement was likely formed by cold water precipitation, resulting in the crypto crystalline (blue agate) structure that yet retains the definitive imprints of growth of ancient microbial mats.