David May, Assistant Chief of Interpretation and Resource Management, Moab office of National Park Service (oral commun., Oct. 1973), believes that if only those in the park having a minimum dimension of 10 feet in any one direction were considered to be arches, the number would boil down to about 56 or 57. The most complete count of arches and other openings in all of southeastern Utah was made by Dale J. Stevens, Professor of Geography at Brigham Young University, during the period February through April 1973. He considered those with openings of 3 feet or larger and found more than 300 in southeastern Utah, of which 124 are in Arches National Park, although he stated that several areas of the park were not intensively searched because of time limitations (written commun., July and Sept. 1973). The 124 arches and openings are distributed among the several named areas of the park, as follows: Courthouse Towers, 13; Herdina Park, 11; The Windows section, 25; Delicate Arch area, 3; Fiery Furnace, 19; Devils Garden, 25; upper Devils Garden (northwest of Devils Garden), 14; Eagle Park, 2; and Klondike Bluffs, 12. Professor Stevens generally used a range finder or a steel tape to measure the width and height of the openings and the width and thickness of the spans, but estimated a few of the dimensions. In the text descriptions of arches or captions of figures that follow, I am including all or part of these measurements, without further acknowledgment. All the arches in the park were formed in the Entrada Sandstone, mainly in the Slick Rock Member but partly in the Slick Rock and Dewey Bridge Members, and a few in the Slick Rock Member occur not far beneath the base of the overlying Moab Member. The sandstone of the three members is composed mainly of quartz sand cemented together by calcium carbonate (CaCO₃), which also forms the mineral calcite and the rock known as limestone, but the Dewey Bridge Member also contains beds of sandy mudstone. Limestone and calcite are soluble in acid, even in weak acid such as carbonic acid, HHCO₃, also written H₂CO₃, formed by the solution of carbon dioxide (CO₂) in water. Ground water, found everywhere in rock openings at different depths beneath the land surface, contains dissolved carbon dioxide derived from decaying organic matter in soil, from the atmosphere, and from other sources. Even rainwater and snow contain a little carbon dioxide absorbed from the atmosphere—enough to dissolve small amounts of limestone or of calcite cement from sandstone. The calcite cement in the Entrada and in many other sandstones is unevenly distributed, however, so that all the cement is removed first from places that contain the least amounts, and, once the cement is dissolved away, the loose sand is carried away by gravity, wind, or water.