Sandstone is a common sedimentary rock primarily composed of sand-sized grains (0.0625 to 2 millimeters in diameter), with quartz and feldspar being the most frequent constituents. Its creation involves the erosion and transportation of source material, followed by the deposition of sand grains in a specific location. The environment where the sand settles dictates the resulting rock’s texture, composition, and internal structure. These distinct characteristics provide geologists with clues about the wind, water, and geography of the past.
Continental Formation Settings
Continental settings are entirely landlocked, where sand transport and deposition are governed by wind, rivers, or ice. The lack of continuous, large bodies of standing water significantly influences the sandstone’s appearance. The resulting rock often preserves features unique to terrestrial processes, such as large-scale dune structures or channel-fill deposits.
Sandstone formed in eolian (desert) environments is characterized by excellent sorting because wind only carries grains of a specific size. These wind-blown deposits, like ancient sand dunes, often consist of well-rounded, fine-grained quartz polished during transport. A defining feature is the presence of large-scale cross-bedding, where layers are inclined at steep angles reflecting the slip faces of migrating dunes.
Fluvial (river) environments produce continental sandstone that is less well-sorted than eolian deposits. River systems, especially fast-moving or braided streams, transport a wider range of sediment sizes. These deposits often contain varied grain sizes, including fragments of clay, silt, or gravel trapped between the larger sand grains. Fluvial sandstone frequently exhibits structures related to channel migration, such as fining-upward sequences, where coarse sand at the base grades upward into finer sediment.
Transitional Formation Settings
Transitional environments exist at the boundary between land and sea, where continental and marine forces interact to shape accumulating sediments. The energy of the water—from waves, tides, and river currents—determines the characteristics of the deposited sand. These areas lead to complex layering and sedimentary structures within the rock record.
Deltaic environments, formed where a river enters a standing body of water (sea or lake), create a complex mix of sand, silt, and mud. Rapid deposition results in poorly sorted sandstone, as the river quickly loses energy and dumps its sediment load. These deposits show a coarsening-upward sequence as the delta progrades (builds outward), with fine mud at the base followed by coarser channel and mouth bar sands.
Sandstone formed along beaches and shorelines is marked by maturity and excellent sorting. The constant back-and-forth action of waves washes away fine mud and silt particles, leaving behind clean, well-sorted sand. This wave action creates distinctive, low-angle, planar stratification that runs parallel to the beach face, reflecting the swash and backwash of the surf.
Tidal flats are found in sheltered areas where the water level fluctuates dramatically. The cyclical exposure and submergence of these flats lead to alternating layers of sand and mud. This sometimes results in herringbone cross-stratification due to the opposing directions of tidal currents. Mud cracks and ripple marks are common features preserved, indicating periodic drying and gentle water movement.
Marine Formation Settings
Marine settings are entirely submerged beneath the ocean surface, where water depth and current mechanisms control the final sandstone characteristics. These environments range from shallow coastal shelves to the deep ocean floor, yielding a diverse suite of sandstone types. The sustained presence of water allows for greater influence from long-term current and biological activity.
The shallow marine shelf, extending from the shoreline to the continental slope, is a common site for sandstone deposition. Sandstone here is often mature and rich in quartz, similar to beach sand, due to long-term reworking by storm waves and steady currents. Characteristic indicators include marine fossils and structures like hummocky cross-stratification, which is formed by large storm waves.
Deep marine environments, such as the continental slope and rise, form sandstone through rapid, gravity-driven flows called turbidity currents. These underwater landslides carry a poorly sorted mixture of sand and mud down the slope, depositing them as fan-shaped bodies on the ocean floor. The resulting sandstone layers, known as turbidites, are characterized by graded bedding, where the coarsest sand is at the base and gradually becomes finer toward the top.
The Final Step: Turning Sand into Stone
Regardless of the deposition environment, the final transformation into solid sandstone requires lithification. This physical and chemical change begins once loose sand grains are buried beneath accumulating sediment. The weight of this material initiates compaction, squeezing the grains closer and reducing the original pore space.
As the pore space decreases, water carrying dissolved minerals flows through the remaining gaps. The second part of lithification, cementation, occurs when these dissolved minerals precipitate out of the water, acting as a glue. Common cementing agents include silica (quartz), calcite, and iron oxides, which bind the sand grains together.
The type of cement impacts the sandstone’s final color and strength; for instance, iron oxides produce the distinct red or brown hues seen in many desert sandstones. This combined process of compaction and cementation transforms unconsolidated sand into durable, solid rock.