The black streaks on the North canyon wall are caused by manganese oxide formation on the surface of the rocks. Exposure to the air causes the Manganese (Mn) in the rock to react with oxygen forming MnO2. This change in the chemical composition of the rocks surface actually breaks the rock down over time, that's why it is called chemical weathering. Notice that some of the rocks pictured below (this is the tunnel from stop #6) appear reddish brown in color. This red brown color is the result of another type of chemical weathering, iron oxidation. When iron reacts with oxygen to form iron oxide the result is commonly known as rust. The red coloring on the rocks is not rust though, it is staining from iron oxide.


Chemical weathering is a much slower process than physical weathering but it is happening almost everywhere!