The conventional concept is that the "banded iron" layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria. This combined with dissolved iron in Earth's oceans to form insoluble iron oxides. These precipitated out, forming a thin layer on the substrate, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve: the banding is assumed to result from cyclic variations in available oxygen.