Comparison of the rock columns for the two parks also reveals other differences. Both parks contain exposures of rocks as old as the Pennsylvanian Paradox Member of the Hermosa Formation. However, only in the Horseshoe Canyon Detached Unit of Canyonlands are rocks as young as the Jurassic Entrada Sandstone, whereas all the spectacular natural arches that make Arches famous were formed in the Entrada Sandstone, and Arches also contains several younger formations of Jurassic and Cretaceous age (fig. 4). A commonly asked question is “Why are most of the rocks so red, particularly those in which the arches were formed?” This can be answered with one word—iron, the same pigment used in rouge and in paint for barns and boxcars. Various oxides of iron, some including water, produce not only brick red but also pink, salmon, brown, buff, yellow, and even green or bluish green. This does not imply that the rocks could be considered as sources of iron ore, for the merest trace, generally only 1 to 3 percent, is enough to produce even the darkest shades of red. The white or nearly white Navajo Sandstone and the Moab Member of the Entrada Sandstone contain little or no iron. As pointed out by Stokes (1970, p. 3), microscopic examination of the colored grains of quartz or other minerals shows the pigment to be merely a thin coating on and between white or colorless particles. Sand or silt weathered from such rocks soon loses its color by the scouring action of wind or water, so that most of the sand dunes and sand bars are white or nearly so. Bending And Breaking of The Rocks Perhaps the greatest geologic contrast between these two closely adjacent parks lies in their different geologic structure—the kind and amount of bending and breaking of the once nearly flat lying strata. Consolidated rocks, particularly brittle types, are subject to two types of fracturing by Earth forces. Joints are fractures along which no movement has taken place. Faults are fractures along which there has been displacement of the two sides relative to one another (fig. 6). As noted in the report on Canyonlands National Park (Lohman, 1974), the strata there, particularly along the valley of the Green River, are virtually flat lying or have only very gentle dips. Along the Colorado River above the confluence with the Green, however, the slightly dipping strata are interrupted by several gentle anticlinal and synclinal folds (fig. 5) and by at least one fault (fig. 6). The largest of these folds—the Cane Creek anticline, which crosses the Colorado River north of Canyonlands—has yielded oil in the past and is now yielding potash by solution mining of salt beds in the Paradox Member of the Hermosa Formation. [Illustration: COMMON TYPES OF ROCK FOLDS. Top, Anticline, or upfold; closed anticlines are called domes. Bottom, Syncline, or downfold; closed synclines are called basins. From Hansen (1969, p. 31, 108). (Fig. 5)]