Oxidation-- Chemical Weathering
This boulder lying in the creek contains additional evidence of chemical weathering by oxidation. The red splotches are actually deposits of ferric iron in the form of the mineral hematite. The iron was derived from minerals in the sediment during an earlier stage of weathering and soil formation. Examples of ancient soils are very common in the Clark Creek area. They are readily seen in the clayey horizons about three to six feet below the lips of the waterfalls. Oxidation responsible for these iron deposits is the same process that causes objects made of iron and steel to rust.
The image below illustrates how hematite (red splotches) appears in an ancient soil horizon exposed in the walls of Clark Creek by erosion. The splotches are typically 2-3 inches in diameter.