banded iron

Alternating stripes of red jasper and silvery-gray hematite. Very heavy for its size.

These bands are the last chapter of the iron formations, not the first. The great oxygenation of the atmosphere was already half a billion years past when these laid down. The great iron formations shut down around 1.85 billion years ago, when sulfide in a still oxygen-free deep ocean began locking iron up as pyrite instead. Iron formation only comes back much later, and briefly, during the snowball-earth glaciations.