In strong contrast to Canyonlands, Arches National Park contains three northwesterly trending major folds and is bordered on the southwest by a fourth. The largest and most important are the collapsed Salt Valley and Cache Valley anticlines, which separate the two most scenic groups of arches and other erosional forms—Eagle Park, Devils Garden, Fiery Furnace, and Delicate Arch on the northeast, and Klondike Bluffs, Herdina Park, and The Windows section on the southwest. Farther southwest is the Courthouse syncline, containing the attractive group of erosional forms called Courthouse Towers (fig. 1). Finally, near the southwest edge of the park, is the Seven Mile-Moab Valley anticline (also known as the Moab-Spanish Valley anticline), whose southwest limb is cut off by the Moab fault (figs. 7, 23). The folds just named and the sharply contrasting geologic structures of the two parks are well shown on sheet 2 of the geologic map of the Moab quadrangle (Williams, 1964), and the geologic formations are shown in color on sheet 1. [Illustration: COMMON TYPES OF FAULTS. Top, Normal, or gravity fault, resulting from tension in and lengthening of the Earth’s crust. Bottom, reverse fault, resulting from compression in and shortening of the Earth’s crust. Low-angle reverse faults generally are called overthrusts or overthrust faults. In both types, note amount of displacement and repetition of strata. Displacements may range from a few inches or feet to many thousands of feet. From Hansen (1969, p. 116). (Fig. 6)] [Illustration: PARADOX BASIN, in southeastern Utah and southwestern Colorado, showing the extent of common salt and major potash deposits in the Paradox Member of the Hermosa Formation, and the salt anticlines. Adapted from Hite (1972, fig. 1B). (Fig. 7)] [Illustration: GEOLOGIC SECTION ACROSS NORTHWEST END OF ARCHES NATIONAL PARK, showing strata beneath Courthouse syncline and Salt Valley anticline. For line of section, see figure 9. Caprock consists of gypsum and shale, from which common salt has been leached by ground water, covered by alluvium. Heavy slanted lines near crest of anticline are faults. Adapted from Hite and Lohman (1973, fig. 13). (Fig. 8)] [Illustration: INDEX MAP OF NORTHWESTERN PART OF ARCHES NATIONAL PARK, showing axes of Courthouse syncline and Salt Valley anticline, line of section _A_-_A_′ in figure 8 and line of section _B_-_B_′ in figure 10. Open circles along line of section are sites of test wells for oil, gas, or potash. Adapted from Hite and Lohman (1973, fig. 12). (Fig. 9)]