Sandstone is a clastic sedimentary rock formed by the cementation of sand-sized mineral fragments. These fragments, known as framework grains, are typically composed of durable minerals like quartz and feldspar. Sandstone is one of the most common rocks on Earth, making up between 20% and 25% of all sedimentary rocks globally. The binding material, or cement, that holds the grains together is usually silica, calcite, or iron oxide, precipitated from water moving through the sediment.
Visual Assessment: Grain Size and Texture
The most immediate way to identify sandstone is by the size and feel of its constituent grains. Sand-sized grains are defined as those between 0.0625 millimeters and 2 millimeters in diameter. Rubbing the rock surface is a definitive tactile test; it should feel distinctly gritty, much like coarse sandpaper.
Observing the texture can reveal the history of the rock’s formation. Sorting refers to the uniformity of the grain sizes. Well-sorted sandstone has grains all roughly the same size, suggesting deposition in a stable environment like a beach or dune. Conversely, poorly-sorted sandstone contains a wide range of grain sizes, often indicating rapid deposition near the source rock.
Roundness describes how worn the edges of the individual grains are. Grains range from angular (sharp, little transport) to well-rounded (smooth, extensive transport). Highly rounded grains suggest a long period of transport by water or wind, such as in ancient desert dune formations. Angular grains are characteristic of rocks deposited quickly near the source, like river floodplains.
Testing Key Physical Properties
While the grains themselves are visually distinctive, testing the rock’s mechanical properties helps confirm its identity. The primary mineral in most sandstones, quartz, has a Mohs hardness of 7. The overall hardness of the rock is highly variable, depending on the strength of the cement holding the grains together.
To perform a field test, attempt to scratch the rock with a steel tool, such as a knife point, which has a hardness of approximately 5.5. If the steel leaves a scratch, the cement is likely calcite or iron oxide, and the rock is relatively soft. If the steel does not scratch the rock, the cement is likely silica, bonding the quartz grains tightly and making the overall rock hardness closer to 7.
Sandstone is characterized by its porosity, containing small spaces between the grains. This can be observed by applying a small amount of water to a fresh, dry surface; a porous sandstone will absorb the moisture relatively quickly, becoming visibly darker. This porosity distinguishes it from many metamorphic rock types.
Identifying Sedimentary Structures
Sandstone frequently preserves large-scale features that provide evidence of its depositional environment. The most basic feature is bedding, or layering, which appears as distinct, parallel, horizontal lines that represent successive depositional events. These layers are separated by bedding planes, marking breaks in sediment accumulation.
A highly diagnostic feature is cross-bedding, which consists of internal layers inclined at an angle to the main horizontal bedding planes. These angled layers form as migrating ripples or dunes deposit sand on their down-current side, and the angle of inclination indicates the direction of the ancient current flow, whether wind or water.
The presence of ripple marks also confirms the formation environment. These wave-like patterns are visible on the bedding planes and are the preserved remnants of ripples formed by moving water or wind on the surface of the loose sand before it was cemented into rock.
Differentiating Sandstone from Look-Alikes
Two common rocks are frequently confused with sandstone, requiring attention to textural details for accurate identification. Quartzite is the metamorphic equivalent of quartz-rich sandstone, formed by intense heat and pressure. The difference is textural: in quartzite, the original quartz grains have been fused together, causing the rock to fracture through the grains, giving the broken surface a smooth, glassy, or sugary sheen.
In contrast, sandstone breaks around the cement, leaving the distinct, individual sand grains visible and giving the rock a rougher, duller fracture surface. Quartzite is also significantly harder and non-porous, making it impervious to water, unlike the relatively porous sandstone.
Sandstone also needs to be distinguished from finer-grained sedimentary rocks like siltstone and shale. Siltstone is composed of grains between 1/16 millimeter and 1/256 millimeter, which are too fine to be easily seen with the naked eye. While siltstone may feel slightly gritty if rubbed, it lacks the distinct, coarse, sandpaper-like feel of sandstone.
Shale is even finer, made of clay-sized particles, and feels completely smooth and slick to the touch, often exhibiting a property called fissility, which causes it to split easily into thin, flat layers.