evidence: Most of the landscape surrounding Devils Tower is composed of sedimentary rocks. These rocks are formed from solidification of minerals or organic material, and are usually deposited by water or wind. This process, known as deposition, is common in river deltas and coastal areas. Many fossils are found in sedimentary rocks, providing us with clues about ancient ecosystems. The oldest rocks visible in Devils Tower National Monument were deposited in a shallow inland sea. This sea covered much of the central and western United States during the Triassic period, 225 to 195 million years ago. Dark red sandstone and maroon siltstone, interbedded with shale, can be seen along the Belle Fourche River. Oxidation of iron-rich minerals causes the red color of the rocks. This rock layer is the Spearfish Formation.