Besides variety of form, sedimentary rocks furnish a wide range of color derived from mineral dyes dissolved out of rocks by erosion. The gorgeous tint of the Vermilion Cliff in Utah and Arizona, the reds and greens of the Grand Canyon and Glacier National Park, the glowing cliffs of the Canyon de Chelly, and the variegated hues of the Painted Desert are examples which have become celebrated.
Geologists distinguish many kinds of sedimentary rocks. Scenically, we need consider only four: limestone, conglomerate, sandstone, and shale.
Limestone is calcium carbonate derived principally from sea-water, sometimes from fresh water, either by the action of microscopic organisms which absorb it for their shells, or occasionally by direct precipitation from saturated solutions. The sediment from organisms, which is the principal source of American scenic limestones, collects as ooze in shallow lakes or seas, and slowly hardens when lifted above the water-level. Limestone is a common and prominent scenic rock; generally it is gray or blue and weathers pale yellow. Moisture seeping in from above often reduces soluble minerals which drain away, leaving caves which sometimes have enormous size.
The other sedimentary rocks which figure prominently in landscape are products of land erosion which rivers sweep into seas or lakes, where they are promptly deposited. The coarse gravels which naturally fall first become conglomerate when cemented by the action of chemicals in water. The finer sandy particles become sandstone. The fine mud, which deposits last, eventually hardens into shale.
Shale has many varieties, but is principally hardened clay; it tends to split into slate-like plates each the thickness of its original deposit. It is usually dull brown or slate color, but sometimes, as in Glacier National Park and the Grand Canyon, shows a variety of more or less brilliant colors and, by weathering, a wide variety of kindred tints.
Sandstone, which forms wherever moving water or wind has collected sands, and pressure or chemical action has cemented them, is usually buff, but sometimes is brilliantly colored.
The processes of Nature have mixed the earth's scenic elements in seemingly inextricable confusion, and the task of the geologist has been colossal. Fortunately for us, the elements of scenery are few, and their larger combinations broad and simple. Once the mind has grasped the outline and the processes, and the eye has learned to distinguish elements and recognize forms, the world is recreated for us.