Sandstone

a rock composed of grains of sand held together by some cement. When grains of silica are bound and stained by iron oxide a brown sandstone is formed. Such sandstones are found at intervals from the Atlantic to Minnesota, and are used for the famous brown sandstone fronts of New York and other cities. Sometimes the binding material is lime or clay, forming lime and clay sandstones. The materials are usually sorted by the action of water. Sandstones frequently show the ripple marks of old shores, and have preserved the tracks of wading birds and other animals long since extinct. Grindstones are made usually of a fine grained sandstone. See Sand; Grindstone