Sedimentary rocks form from the accumulation and cementation of fragments derived from pre-existing rocks, minerals, or once-living organisms. These materials, known as sediment, are typically deposited in layers on the Earth’s surface or under water bodies. Sediment is transported by agents like water, wind, or ice until it settles in a basin. Deep burial subjects the sediment to compaction and cementation, hardening the loose material into solid rock, a process called lithification. Field identification focuses on recognizing the physical and chemical clues that record the environments where the rocks originally formed.
Step 1: Identifying Stratification and Surface Structures
The most immediate visual clue for a sedimentary rock is layering, known as stratification or bedding. These layers represent successive episodes of deposition, separated by a bedding plane that marks a pause or change in sedimentation. The layers are often horizontally oriented, reflecting the law of superposition where younger layers are deposited atop older ones.
Observing the size and geometry of these beds offers insight into the depositional environment. Layers inclined relative to the main bedding planes, called cross-bedding, often form from the migration of ancient dunes or ripples. Graded bedding is another structure where grain size within a single layer systematically decreases from bottom to top.
Surface features preserved on the top of a layer offer specific environmental details. Ripple marks, which look like small parallel waves, indicate the action of currents in shallow water or wind. Mud cracks suggest a past environment that was periodically submerged and then dried out, such as a tidal flat. Trace fossils, like ancient tracks or burrows, further confirm a sedimentary origin as they are preserved remains of biological activity.
Step 2: Analyzing Grain Characteristics and Cementation
After confirming layering, the next step is examining the rock’s texture, specifically the size, shape, and arrangement of its constituent fragments, or clasts. Clastic sedimentary rocks, formed from mechanically weathered fragments, are classified primarily by grain size.
Estimating grain size involves distinguishing between four main categories:
- Clay (too small to see).
- Silt (feels gritty when rubbed).
- Sand (visible and gritty to the touch).
- Gravel (fragments larger than 2 millimeters).
The degree of sorting, or the uniformity of grain sizes, indicates the energy and duration of transport. Well-sorted rocks, where grains are roughly the same size, suggest prolonged transport by a consistent agent like wind or water. Conversely, a poorly-sorted rock, containing a wide mix of sizes, indicates rapid deposition close to the source, such as a debris flow.
Grain shape is also a useful indicator, comparing rounding versus angularity. Highly rounded grains suggest significant abrasion during long-distance transport, while angular grains imply minimal movement from the source rock. These fragments are bound together by a cementing material that precipitates from water circulating through the pore spaces.
Common cements include silica, calcite, and iron oxides, which influence the rock’s hardness and color. Silica cement creates a very hard rock. Calcite cement will cause the rock to fizz when tested with dilute acid.
Step 3: Categorizing by Composition and Origin
The final step involves categorizing the rock into one of three main genetic groups based on composition and origin.
Clastic Sedimentary Rocks
Clastic sedimentary rocks are defined by their fragmental texture. Examples include Sandstone (sand-sized grains), Shale (clay-sized grains that split easily), and Conglomerate or Breccia (gravel-sized fragments). Conglomerate contains rounded pebbles, while Breccia has sharp, angular clasts, indicating different transport histories.
Chemical Sedimentary Rocks
Chemical sedimentary rocks form when minerals precipitate directly out of a water solution, often due to evaporation. Limestone, composed of calcite, is the most common example and reacts vigorously to dilute hydrochloric acid. Evaporites, like Gypsum or Halite (rock salt), are soft and easily identified by their mineral properties; Gypsum can be scratched with a fingernail.
Organic Sedimentary Rocks
Organic sedimentary rocks form from the accumulation of material derived from once-living organisms. Coal, a black, combustible rock, is formed from compacted plant matter. Certain types of Limestone are also organic, forming from the shells and skeletal remains of marine organisms, such as Chalk, which is made of microscopic marine shells.