File:Iron banding 01.jpg

English: An ashtray carved out of banded ironstone, showing the clear red-grey banding that resulted from photosynthetically produced oxygen reacting with soluble iron (II) compounds dissolved in water to form red, insoluble iron oxide, which precipitated out to form the red bands. The alternance of red and grey bands is not due to short day/night cycles related to the photosynthesis activity. It has a longer period and is related to much longer cycles, such as these of varve rhythmic deposits, release of soluble Fe²⁺ at the mid-ocean ridges, or biological cycles of the cyanobacteria (starved by the depletion of Fe²⁺ ions, or poisoned by some by-products of their metabolism. The mesobanding structure could also be due to segregation processes occurring during mud sediments deposition and compaction. A few millimeters thick grey band cannot form during night: it would be a considerably high sedimentation rate. If it was the case, about 2 meters sediments (a considerable thickness) would settle each year which would correspond to a few kilometers sediments on a period of one million years. This is not realist and not compatible with best estimations.