The curious red rock formations you see at Papago Park are nature's answer to modern art. Here, the sandstone has been sculpted by millions of years of weathering, as the process of breaking down rocks is known. The holes or cave-like recesses called tafoni are created by salt, which expands when heated by the hot desert sun and, over time, causes small pieces of rock to chip away. Erosion, the movement of rock by water and wind, takes the pieces that have broken off away. Yet weathering and erosion aren't the only natural phenomena that make the rocks in Papago Park interesting. Oxidation, the change that occurs when oxygen reacts with minerals, is responsible for their color. When iron and oxygen combine to form hematite they turn the rock a deep red.