Sedimentary Rock Types and Definitions Quiz

Chert forms from microcrystalline quartz, giving it a very fine texture and a glass-like fracture. Its silica-rich composition makes it hard and smooth, often breaking with sharp edges. Chert can form chemically in deep marine settings or through replacement processes within limestone. These features help distinguish it from volcanic glass or granite, which form from molten materials rather than chemical or sedimentary processes.

Shale is a sedimentary rock made of compacted clay and silt-sized particles. Its defining feature is its thin layering, known as fissility, which allows it to break along flat planes. Because the grains are too small to see, shale appears smooth and dull. It typically forms in quiet water environments such as lakes, deep oceans, or river floodplains, where fine sediments slowly accumulate in layers.

Coal is a sedimentary rock formed from accumulated plant material buried in swamps and compacted over millions of years. As heat and pressure increase, the carbon content rises, producing lignite, bituminous coal, and eventually anthracite. Coal is organic in origin, unlike igneous or metamorphic rocks. Its black color, lightweight feel, and ability to burn help distinguish it from other rock types.

Coquina is a sedimentary rock made almost entirely of shell fragments held together by natural cement. Its texture is rough and porous, resembling broken seashells glued together. Coquina forms in high-energy coastal environments where waves deposit shell debris. This composition makes it easy to identify and separates it from crystalline limestone or igneous rocks.

Conglomerate contains large, rounded clasts (over 2 mm) cemented together by finer material. The roundness of the fragments indicates they were transported by rivers or waves before deposition. This helps differentiate conglomerate from breccia, which has angular fragments. Its mixed composition and coarse texture reflect a high-energy depositional environment, such as fast-moving streams.

Sandstone forms from cemented sand-sized grains, typically quartz or feldspar. Its gritty feel and visible grains make it easy to recognize. Sandstone forms in environments such as beaches, deserts, or river channels where sand accumulates. The mineral composition and grain size help distinguish sandstone from shale, which has finer grains, and from limestone, which reacts with acid.

Silt refers to fine sediment particles smaller than sand but larger than clay. Silt-sized grains create rocks with a smooth texture but slightly gritty feel when rubbed. These grains are often transported by wind or slow-moving water and settle in calm environments. Silt is a size category, not a specific rock, but it helps identify siltstone and other fine-grained sedimentary rocks.

Fossils form when organisms are buried rapidly in sediment, making sedimentary rocks the best environment for preservation. Heat in igneous formation destroys organic remains, and metamorphic processes distort fossils beyond recognition. Sedimentary rocks create gentle burial conditions that allow shells, bones, and plant material to be preserved. Fossils help interpret past climates, environments, and life forms.

Coal forms when plant remains accumulate in swampy environments, become buried, and undergo chemical changes during compaction. Over time, pressure drives out moisture and gases, increasing carbon content. This organic origin distinguishes coal from igneous or metamorphic rocks. Coal’s lightweight structure and ability to burn make identification straightforward.

Breccia is a sedimentary rock composed of large, angular fragments cemented together. The sharp edges indicate the fragments were not transported far, unlike the rounded pieces of conglomerate. Breccia often forms near fault zones, landslides, or volcanic activity where rocks are broken apart rapidly. The angular texture is its key identifying feature.