Did you know that the Arb is home to evidence of early microbial life? A large ancient stromatolite is visible along Spring Creek in the Upper Arb. Stromatolites are microbial reefs created by the activity of cyanobacteria.

Microbial mats, or complex communities of microbes (primarily bacteria and archaea) organized into visible layers, were one of the first ecosystems on Earth. The oxygen-rich air created by ancient photosynthetic mats was crucial to the development of complex life on Earth. Stromatolites are mineralized microbial mat fossils, and hold a record of ancient life on Earth.

Stromatolites feature lamination, or layering, which is formed when calcium carbonate precipitates over a microbial mat and hardens. This process happens because of microbial photosynthesis, which depletes the supply of carbon dioxide in the surrounding water. As this continues to happen over time, more and more layers are built up on top of each other and the stromatolite increases in size. These same structures are still around today and can be found in shallow marine seas, most notably in the Bahamas and Western Australia.

While the oldest found stromatolites are 3.5 billion years old (the Earth is 4.5 billion years old), the stromatolites visible next to Spring Creek in the Upper Arb are located in the Prairie du Chien group of sedimentary rocks, which are dated to the early Ordovician period (about 488 million years ago). While 488 million years is still pretty ancient by most standards, that’s nearly 3 billion years younger than the first stromatolites!

The stromatolite visible in the Upper Arb consists of smaller laminated stromatolites as part of a larger meter-tall composite structure. Stop by and investigate for yourself.

NASA believes microbial mats are central to the search for evidence of life both on Earth and in space.