Differences in the composition, hardness, arrangement, and thickness of the rock layers determine their ability to withstand the forces of fracturing and erosion and, hence, whether they tend to form cliffs, ledges, fins, or slopes. Most of the cliff- or ledge-forming rocks are sandstones consisting of sand deposited by wind or water and later cemented together by silica (SiO₂), calcium carbonate (CaCO₃), or one of the iron oxides (such as Fe₂O₃), but some hard, resistant ledges are made of limestone (calcium carbonate). The rock column ([fig. 4]) shows in general how these rock formations are sculptured by erosion and how they protect underlying layers from more rapid erosion. The nearly vertical cliffs along the lower reaches of Salt and Courthouse Washes and the Colorado River canyon upstream from Moab consist of the well-cemented Wingate Sandstone protected above by the even harder sandstones of the Kayenta Formation. (See figs. [21], [22].) To borrow from an earlier report of mine (Lohman, 1965, p. 17), “Vertical cliffs and shafts of the Wingate Sandstone endure only where the top of the formation is capped by beds of the next younger rock unit—the Kayenta Formation. The Kayenta is much more resistant than the Wingate, so even a few feet of the Kayenta * * * protect the rock beneath.” In some places, as shown in figures [19] and [20], the overlying Navajo Sandstone makes up the topmost unit of the cliff.

JOINTED NORTHEAST FLANK OF SALT VALLEY ANTICLINE, viewed westward from an airplane. Light-colored wedge in middle background is Salt Valley bordered on extreme left by Klondike Bluffs. Dark-colored fins and pinnacles on left, of Slick Rock Member of the Entrada Sandstone, form Devils Garden. Sharp pinnacle above valley is the Dark Angel. (See [fig. 57].) White bands of sandstone extending to foreground are composed of Moab Member of the Entrada. Note vegetation in the joints. Photograph by National Park Service. (Fig. 12)

Last but far from least among the factors responsible for the grandeur of Arches National Park and the Plateau in general is the desert climate, which allows one to see virtually every foot of the vividly colored naked rocks, and which has made possible the creation and preservation of such a wide variety of fantastic sculptures. A wetter climate would have produced a far different, smoother landscape in which most of the rocks and land forms would have been hidden by vegetation. On the Plateau the vegetation grows mainly on the high mesas and the narrow flood plains bordering the rivers, but scanty vegetation also occurs on the gentle slopes or flats.

The combination of layers of sediments of different composition, hardness and thickness, the bending and breaking of the rocks, and the desert climate, has produced steep slopes having many cliffs, ledges, and fins with generally sharp to angular edges, rather than the subdued rounded forms of more humid regions.

Origin And Development of The Arches

Among the questions commonly asked by visitors are, “How do arches form?”, “Why are some openings called windows, others arches?”, “What is the difference, if any, between arches or windows and natural bridges, such as those at Natural Bridges National Monument?”, and “How many arches are there in Arches National Park?” Before taking up the origin and development of arches, I shall attempt to explain the differences between the three types of natural rock openings named above and comment upon the number of arches.

INDEX MAP, showing localities where most of the photographs were taken. Arrows point to distant views. Numbers refer to figure numbers. (Fig. 13)
[High-resolution Map]