Sandstone

Sandstone is a clastic sedimentary rock composed of sand-sized grains (0.0625 to 2 mm) cemented together, most commonly dominated by quartz, with varying amounts of feldspar and rock fragments. It is one of the most common and recognizable sedimentary rocks.

Its color depends on its grains and cement: iron oxides produce red and brown hues, while pure quartz sands yield white or tan stone. The space between grains (porosity) makes many sandstones important aquifers and petroleum reservoirs.

Sandstone often preserves sedimentary structures like cross-bedding and ripple marks, recording the rivers, deserts, deltas, and shallow seas in which the sand was originally deposited.

Sandstone forms when sand grains, eroded from older rocks and transported by water or wind, accumulate in environments such as beaches, deserts, riverbeds, deltas, and shallow marine shelves. Over time the grains are buried, compacted, and cemented by minerals like silica, calcite, or iron oxide deposited from groundwater.

The maturity of a sandstone (how well-rounded and well-sorted its grains are, and how pure the quartz) reflects how far and how long the sand traveled before deposition.

Famous sandstones include the Navajo and Entrada formations of the American Southwest (Zion, Antelope Canyon), the red sandstones of Petra in Jordan, England's Old Red Sandstone, and the Uluru monolith in Australia.

Look for a gritty rock made of visible sand grains you can often rub loose with a finger. It typically feels like sandpaper and may show layering, cross-bedding, or ripple marks.

Grain size is sand-scale (visible but small), distinguishing it from finer siltstone and shale and from coarser conglomerate. Color ranges widely; red and brown indicate iron oxide cement. A drop of dilute acid will fizz if the cement is calcite.

Look-alikes include siltstone (smoother, grains too small to see), quartzite (the metamorphosed, much harder equivalent that breaks through grains rather than around them), and conglomerate (gravel-sized clasts). The visible, sandy, often friable grain texture is the key identifier.

Sandstone has been a premier building stone for millennia, used in everything from cathedrals and the brownstones of New York to ancient temples, because it is attractive, abundant, and easy to cut ('freestone' cuts in any direction). It is also crushed for aggregate and used as paving and flagstone.

Pure quartz sandstone is a source of silica sand for glassmaking, foundry molds, and hydraulic fracturing 'frac sand'. Its porosity makes sandstone a crucial groundwater aquifer and oil and gas reservoir rock.

Geologically, sandstones are prized archives of past environments and contain abundant fossils and trace fossils that document ancient life and landscapes.