Sandstone is sand cemented together into rock. This is easy enough to tell just by looking closely at a rock. But what does sandstone tell us?
Sandstone is a category of rock made from sediment (a sedimentary rock). The sediment particles are clasts, or pieces, of minerals and fragments of rock, thus sandstone is a clastic sedimentary rock. It is composed mostly of sand, which means particles of a medium size, so sandstone is a medium-grained clastic sedimentary rock. More precisely, sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). Sandstone may include finer and coarser material and still be called sandstone, but if it includes more than 30 percent grains of gravel, cobble or boulder size it's classified instead as conglomerate or breccia (together these are called rudites).
Sandstone has two different kinds of material in it besides the sediment particles: matrix and cement. Matrix is the fine-grained stuff (silt and clay size) that was in the sediment along with the sand whereas cement is the mineral matter, introduced later, that binds the sediment into rock.
Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is called a wacke ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite. Another way to look at it is that wacke is dirty and arenite is clean.
You may notice that none of this discussion mentions any particular minerals, just a certain particle size. But in fact, minerals matter.
Minerals of Sandstone
Sandstone is formally defined strictly by particle size, but rocks made of carbonate minerals don't qualify as sandstone. Carbonate rocks are called limestone and given a whole separate classification, so sandstone really signifies a silicate-rich rock. (A medium-grained clastic carbonate rock, or "limestone sandstone," is called calcarenite.) This division makes sense because limestone is made in clean ocean water, whereas silicate rocks are made from sediment eroded off the continents.
Mature continental sediment consists of a handful of surface minerals, and sandstone therefore is usually almost all quartz. Other minerals—clays, hematite, ilmenite, feldspar, amphibole and mica— and small rock fragments (lithics) as well as organic carbon (bitumen) add color and character to the clastic fraction or the matrix. A sandstone with at least 25 percent feldspar is called arkose. A sandstone made of volcanic particles is called tuff.
The cement in sandstone is usually one of three materials: silica (chemically the same as quartz), calcium carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no matrix.
Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to nearly black, with gray, brown, red, pink and buff in between.
How Sandstone Forms
Sandstone forms where sand is laid down and buried. Usually this happens offshore from river deltas, but desert dunes and beaches can leave sandstone beds in the geologic record too. The famous red rocks of the Grand Canyon, for instance, formed in a desert setting. Sandstone does not usually contain good fossils because the energetic environments where sand beds form don't favor their preservation.
When sand is deeply buried, the pressure of burial and slightly higher temperatures allow minerals to dissolve or deform and become mobile. The grains become more tightly knit together, and the sediments are squeezed into a smaller volume. This is the time when cementing material moves into the sediment, carried there by fluids charged with dissolved minerals. Oxidizing conditions lead to red colors from iron oxides, while reducing conditions lead to darker and grayer colors.
What Sandstone Says
The sand grains in sandstone give information about the past:
- The presence of feldspar and lithic grains means that the sediment is close to the mountains where it arose.
- Detailed studies of sandstone give insight into its provenance—the kind of countryside that produced the sand.
- The degree to which the grains are rounded is a sign of how far they were transported.
- A frosted surface is generally a sign that sand was transported by wind—that in turn means a sandy desert setting.
Various features in sandstone are signs of the past environment:
- Ripples can indicate the local water currents or wind directions.
- Load structures, sole marks, rip-up clasts and similar features are fossil footprints of ancient currents.
- Liesegang bands are signs of chemical action after burial of the sand.
The layers, or bedding, in sandstone are also signs of the past environment:
- Turbidite sequences point to a marine setting.
- Crossbedding (truncated, tilted sandstone layering) is a rich source of information on currents.
- Interbeds of shale or conglomerate may indicate episodes of different climate.
More About Sandstone
As a landscaping and building stone, sandstone is full of character, with warm colors, and can be quite durable. The majority of sandstone quarried today is used as flagstones. Unlike commercial granite, commercial sandstone is the same as what the geologist says it is.
Sandstone is the official state rock of Nevada.
See more sedimentary rocks in the Sedimentary Rocks Gallery.