First among these events was the piling up, layer upon layer, of thousands of feet of sedimentary rocks under a wide variety of environments. Sedimentary rocks of the region are composed of clay, silt, sand, and gravel carried and deposited by moving water; silt and sand transported by wind; and some materials precipitated from water solutions, such as limestone (calcium carbonate), dolomite (calcium and magnesium carbonate), gypsum (calcium sulfate with some water), anhydrite (calcium sulfate alone), common salt (sodium chloride), potash minerals, such as potassium chloride, and a few other less common types. Some of the beds were laid down in shallow seas that once covered the area or in lagoons and estuaries near the sea. Other beds were deposited by streams in inland basins or plains, a few were deposited in lakes, and the constituents of deposits like the Navajo Sandstone, were carried in by the wind. The character and thickness of the exposed sedimentary rocks and the names and ages assigned to them by geologists are shown in the rock column (fig. 4) and in the cross section (fig. 8). The history of their deposition is summarized on pages 98-102. Figure 4 was compiled mainly from generalized sections given by A. A. Baker (1933), Dane (1935), McKnight (1940), and Wright, Shawe, and Lohman (1962), and, in part, from Hite and Lohman (1973). [Illustration: ROCK COLUMN OF ARCHES NATIONAL PARK. Average thickness of units 250-1,000 feet is exaggerated two times; those less than 250 feet, four times. 1 foot = 0.305 meter. (Fig. 4)] AGE (millions of yrs ago) GEOLOGIC AGE NAME OF ROCK UNIT KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION THICKNESS (feet) NAMED FOR OCCURRENCE AT OR NEAR