: a sedimentary deposit that consists of alternating thin layers of iron oxides (such as magnetite or hematite) and iron-poor minerals (such as shale or chert)
That bedrock includes intriguing structures known as banded iron formations, which are believed to occur only with the help of living organisms.— Kenneth Chang, New York Times, 2 Dec. 2008 Note:
Banded iron formations, which formed over a billion years ago, are often used today as a source of low-grade iron ore.
—abbreviation BIF