How lucky am I? Some of the oldest rocks on Earth lie north of my location on Lake Michigan. A half-dozen glacial waves carried rocks from the Precambrian exposures in the northern Great Lakes to my beach in southeastern Wisconsin. Characterized by stripes of red iron-oxide particles, these rocks are called Banded Iron Formation (BIF).
Early Earth had no atmosphere. No lovely layer of the oxygen that all life as we know it depends. Photosynthetic organisms were making oxygen, but that oxygen reacted with iron dissolved in seawater. Resultant iron-oxide minerals fell to the ocean floor, and over time, created Banded Iron Formation, a sedimentary rock. These often brightly colored BIFs date from the Proterozoic Eon and are approximately 2 billion years old. Not only are they some of the oldest rocks on Earth, but they are also some of the most visually stunning.