Sedimentary rocks form when fragments of pre-existing rock, minerals, or organic matter (sediment) accumulate on the Earth’s surface and undergo lithification—the process of turning into rock through compression and cementation. Although they make up only a small fraction of the Earth’s crust by volume, sedimentary rocks cover approximately 73% of the continents’ surface area, forming a thin veneer over older basement rock. Understanding where these rocks originate and how they are exposed is the first step in finding and identifying them in the field.
Primary Geological Environments for Formation
Depositional environments determine the type of sediment that settles and the resulting rock. Marine environments are particularly important; the deep ocean floor typically accumulates fine clay and tiny skeletons of microorganisms, which eventually form shale or deep-sea limestones. Shallow marine settings, such as continental shelves, are warmer and higher-energy, making them ideal for the formation of carbonate rocks like limestone from the accumulation of shells and coral fragments.
Transitional environments exist where land meets the sea, like deltas, beaches, and tidal flats, and they are characterized by alternating energy levels. In these areas, wave and current action sorts sediment, often leading to the deposition of sand-sized grains that become sandstone. Tidal flats, which are alternately exposed and submerged, often preserve features like mud cracks and ripple marks in fine-grained layers.
Continental environments cover all areas on land, including rivers, lakes, deserts, and swamps, each yielding different rock types. Fast-moving mountain streams deposit coarse, poorly-sorted material that lithifies into conglomerate or breccia near the source. Low-energy settings like lakes and swamps accumulate fine clay and organic material, resulting in mudstone, shale, or coal. Deserts are characterized by wind-blown sand, which forms highly-sorted, cross-bedded sandstones.
Large-Scale Landforms Exposing Sedimentary Layers
Sedimentary rocks become visible through uplift and erosion, which create large, distinct landforms. Canyons and river valleys are the most dramatic exposures, as the downcutting action of water removes overlying material, revealing the distinct horizontal layers, or strata. The Grand Canyon, for example, is a vast cross-section of layered rocks, showing formations deposited over millions of years.
Plateaus and mesas are characteristic landscapes of exposed sedimentary layers, often found in arid regions. These flat-topped features result from differential erosion, where a resistant layer of sedimentary rock, such as sandstone or limestone, protects the softer layers beneath it.
Sedimentary basins are large, bowl-shaped depressions where extremely thick sequences of sediment have accumulated over geologic time. While the deepest parts of these basins are often underground, their edges and surrounding uplifts can expose the layers to the surface, offering regional views of the rock record. These basins are economically important but also provide broad areas for geological study.
More accessible, localized exposures can be found along coastal cliffs and road cuts where human activity or natural processes have sliced through the rock. Coastal cliffs expose the layers being eroded by wave action, while highway construction often creates fresh, vertical faces of rock. These smaller exposures provide excellent opportunities to observe the layering and texture of the strata up close.
Practical Tips for Field Identification
Once a potential outcrop is found, several physical characteristics help confirm the rock is sedimentary. The most reliable indicator is the presence of distinct layering, known as bedding or stratification, which appears as parallel bands of varying color or texture.
Sedimentary rocks are the only rock type where fossils are typically found, as the high heat and pressure involved in forming igneous and metamorphic rocks destroy organic remains. The presence of visible shells, plant impressions, or trace fossils is a strong clue that the rock is sedimentary. This is particularly true for limestones, which are often composed entirely of biogenic material.
Clastic sedimentary rocks, such as sandstone and conglomerate, are identified by a texture composed of cemented-together fragments, or clasts. Sandstone feels gritty when rubbed, a result of the visible sand-sized grains, while conglomerate contains rounded pebbles and cobbles. Observing the size and shape of these grains helps determine the rock type and the energy of the ancient environment.
Generally, sedimentary rocks are softer than most igneous or metamorphic rocks because they are held together by cement rather than interlocking crystals. Many sedimentary rocks, particularly shale and siltstone, can be easily scratched with a steel tool or even a fingernail. Carbonate rocks, like limestone, can be further identified by testing for a reaction with a weak acid, which causes fizzing due to the presence of calcium carbonate.