G. William Fiero

"Deposition of limestones, shales, and sandstones created many diverse materials ideal for the sculptor. Earth movements arranged these materials in an interesting manner, resulting in folds, faults, and joints. But the beauty seen in the rocks of Valley of Fire is chiefly the result of that great sculpturing process of geology - erosion.
"Chemical erosion has altered original materials and created brilliant colors in the rocks. The addition of reds by rusting iron minerals, the creation of white sandstones by the leaching out of iron, and the mixing of all shades on the palette of chemical change have resulted in the colorful diversity we see today.
"Chemical action has added soluble materials - lime and silica - to the groundwater. Moving through the joints and fault surfaces of Aztec sandstone, these chemicals occasionally find an environment for precipitation. The joints are sometimes filled with these chemicals. After precipitation the newly formed minerals of calcite or quartz, often harder than the surrounding sandstone, are left standing as narrow ridges after the sandstone has eroded away. Where blocks have sheared off, the vertical joints thus exposed are often veneered with the white mineral deposits.
"Other joint surfaces are coated with a black substance. This 'desert varnish' results from chemical action, perhaps further modified by microscopic plants. Leached from the rocks by the movement of water, iron and manganese are deposited on the rock surface as water emerges and evaporates over thousands of years. These blackened surfaces were favored by the first inhabitants of Valley of Fire as ideal sites for carving rock art." [pp. 17,20]… (altro)