Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray, and white. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone may be strongly identified with certain regions. For instance, much of the North American West is well-known for its red sandstones. Amherst, Ohio is known as the Sandstone Capitol of the World.
Some sandstones are resistant to weathering, yet are easy to work. This makes sandstone a common building and paving material. Because of the hardness of the individual grains, uniformity of grain size, and somewhat friable nature, sandstone is an excellent material from which to make grindstones for sharpening blades and other implements. Non-friable sandstone can be used for grindstones grinding grain.
Rock formations that are primarily sandstone usually allow percolation of water and are porous enough to store large quantities, making them valuable aquifers. Fine grained aquifers, such as sandstones, are more apt to filter out pollutants from the surface than are rocks with cracks and crevices such as limestones or other rocks fractured from seismic activity.
Sandstones are clastic in origin (as opposed to organic, like chalk or coal, or chemical, like gypsum and jasper). They are formed from the cemented grains that may be fragments of a pre-existing rock, or else just mono-minerallic crystals. The cements binding these grains together are typically calcite, clays and silica. Grain sizes in sands are in the range of 0.1 mm to 2 mm. (Rocks with smaller grainsizes include siltstones and shales and are typically called argillaceous sediments, as are also clays. Rocks with larger grainsizes include both breccias and conglomerates and are termed rudaceous sediments.).
The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation of it from either water, as in a river, lake, or sea, or from air, as in a desert. Typically sedimentation occurs by the sand either settling out from suspension; ceasing to be rolled or bounced along the bottom of a body of water or ground surface in a desert; or more typically a combination of both processes. Finally, after once it has accumulated, the sand becomes sandstone when it is cemented by the precipitation of minerals within the pore spaces between sand grains. The most common cementing materials are silica and calcium carbonate, which often derived from either the dissolution or alteration of the sand after it was buried. Colors will usually be tan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand). A predominant additional colorant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red (terra cotta), with additional manganese imparting a purplish hue. Deposition from sand dunes can recognized by irregular and fluidly shaped weathering patterns and wavey coloration lines when sectioned, while water deposition will form more regular blocks when weathered. The regularity of the latter favors use as a source for use in masonry, either as a primary building material or as a facing stone over other construction.
The environment of deposition is crucial in determining the characteristics of the resulting sandstone, which on a finer scale include its grain size, sorting, composition and on a larger scale include the rock geometry and sedimentary structures. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings:
- Terrestrial environments
1. Rivers (levees, point bars, channel sands)
2. Alluvial fans
3. Glacial outwash
5. Deserts (sand dunes and ergs)
- Marine environments
2. Beach and shoreface sands
3. Tidal deltas, flats
4. Offshore bars and sand waves
5. Storm deposits (tempestites)
6. Turbidites (submarine channels and fans)
Types of sandstone
Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups:
arkosic sandstones, which have a high (>25%) feldspar content and a composition similar to granite.
quartzose sandstones which have a high (>90%) quartz content. Sometimes these sandstones are termed "orthoquartzites", e.g., the Tuscarora Quartzite of the Ridge-and-valley Appalachians. Also known as beach sand.
argillaceous sandstones, such as greywacke, which have a significant clay or silt content.