Sedimentary rock
Sedimentary rock forms from particles, minerals, organic material, or chemical precipitates deposited at or near Earth surface. It often preserves layers, fossils, sedimentary structures, and environmental clues that help geologists reconstruct rivers, deserts, lakes, coastlines, oceans, and ancient climates.
What sedimentary rock is
Sedimentary rock is one of the three broad rock groups, along with igneous and metamorphic rock. It forms from material that accumulates at Earth surface or in water. That material may be rock fragments, mineral grains, shells, plant remains, mud, sand, dissolved ions, or organic matter.
From sediment to rock
Loose sediment becomes rock through lithification. Burial adds pressure, compacting grains and squeezing out water. Minerals can precipitate between grains and cement them together. Mud may compact into shale, sand may become sandstone, and gravel may become conglomerate or breccia depending on grain shape and history.
Clastic rocks
Clastic sedimentary rocks are made of fragments of older rocks or minerals. Grain size is one of the main clues: shale is dominated by very fine particles, sandstone by sand-sized grains, and conglomerate or breccia by larger pieces. Grain shape, sorting, and composition reveal transport distance, energy, and source rocks.
Chemical and biochemical rocks
Some sedimentary rocks form when dissolved material precipitates from water. Evaporites such as halite and gypsum can form as salty water evaporates. Limestone may form from calcium carbonate shells, reef material, or direct precipitation. Chert can form from silica-rich microscopic remains or chemical processes.
Layers and structures
Sedimentary rocks commonly preserve bedding, cross-bedding, ripple marks, mud cracks, graded beds, and other structures. These features help geologists infer water depth, current direction, flow strength, drying, storms, floods, dunes, deltas, and other depositional conditions.
Fossils and environments
Many fossils are found in sedimentary rocks because burial in sediment can protect shells, bones, leaves, tracks, burrows, pollen, and other traces from complete destruction. Fossils are not present in every sedimentary rock, but when they occur, they can help identify ages, ecosystems, climates, and depositional settings.
Basins and plate tectonics
Sedimentary rocks often accumulate in basins, where space is available for sediment to pile up. Basins may form along rifts, passive margins, forelands beside mountain belts, deltas, ocean trenches, lakes, and subsiding continental interiors. Plate tectonics helps create the topography, subsidence, uplift, and sediment supply that shape these basins.
Why it matters
Sedimentary rock matters because it is the main archive of Earth surface history. It records changing landscapes, sea level, climate, life, erosion, and water chemistry. It also hosts groundwater, coal, petroleum systems, salt, limestone, building stone, and many records used to understand hazards and environmental change.