A glance at the southeast corner of the map (fig. 1) shows that most of the arches and prehistoric ruins in the park are in Salt Canyon and its main tributary, Horse Canyon. A few are in adjacent Davis and Lavender Canyons, whose headwaters were recently annexed to the park. These canyons are accessible only by negotiating the streambeds on four-wheel-drive vehicles, horseback, or foot. Salt or Horse Canyons are best conquered by four-wheel-drive vehicles plus short hikes in the northern part and long hikes in the southern part. An aerial view (fig. 40) eastward across Salt Canyon shows that erosion has produced an intricate series of meandering canyons separated by rather narrow walls of the Cedar Mesa Sandstone, resembling somewhat The Maze, west of the Green River. [Illustration: AERIAL VIEW EASTWARD ACROSS SALT CANYON. Note narrow walls and pinnacles between canyons and alcoves. Six-Shooter Peaks are in left background. Photograph by Wayne Alcorn, National Park Service. (Fig. 40)] The massive sandstone beds of the Cedar Mesa are composed of sand grains cemented together by calcium carbonate (CaCO₃), which also forms the mineral called calcite and the rock known as limestone. Limestone and calcite are soluble in acid, even weak acid such as carbonic acid (H·HCO₃), formed by solution of carbon dioxide (CO₂) in water. Ground water, found everywhere in rock openings at differing depths beneath the surface, contains considerable dissolved carbon dioxide derived from decaying organic matter in soil, from the atmosphere, and from other sources. Even rain water and snow contain small amounts absorbed from the atmosphere—enough to dissolve small amounts of limestone or of calcite cement in sandstone. The calcite cement in the Cedar Mesa and many other sandstones is unevenly distributed, so the cement is removed first from places that contain the least amounts, and once the cement is dissolved, the loose sand grains are carried away by gravity, wind, or water. Thus, relatively thin walls of sandstone containing irregularly distributed patches of soluble cement are prime targets for the formation of potholes (fig. 46), alcoves, and caves. Once a breakthrough occurs, weakened chunks from the ceiling tend to fall off, and arches of various shapes are produced, because an arch is naturally the strongest form that can support the overlying rock load. Man, from the ancient Greeks, Romans, and Egyptians to modern day, has long made use of arches in building bridges, aqueducts, temples, cathedrals, and other enduring edifices. All the spectacular arches we are about to see were carved from the Cedar Mesa Sandstone.