English: Petrified Forest National Park, Arizona -- The pastel bands of color in the Blue Mesa indicate the type of environment in which the sediments were deposited. Blues and grays are a sign of carbon from decaying organic material that was buried or under water and not exposed to air. Reds result from an iron mineral, hematite. When exposed to air, even a small amount of iron causes the rocks to oxidize or rust. The whites represent nearly pure bentonite clay. Ground water, percolating through the buried sediments, also accounts for some color changes.