Liesegang weathering

Precipitation from solution fronts, a natural analog of chromatography. The red mineral is hematite (ferric oxide), with some orange goethite (iron oxyhydroxide, better known as rust). This sort of weathering is very common in arid environments. Aztec Sandstone, southern Nevada, USA. The coin is there to show scale and is about 1 inch (2.5 cm) across.