By Laura Wormuth, Updated Aug 30, 2022
There are two main categories of sedimentary rocks. The first, formed by chemical precipitation, includes limestone and chert. The second—detrital or clastic rocks—are composed of mineral fragments that become lithified through compaction and cementation as they accumulate in layers.
Shale forms when the finest clay‑sized particles settle in calm water bodies such as deep lakes or the ocean floor. Over time, the weight of overlying sediments compresses the clay, producing a fine‑grained rock that often splits into thin, parallel layers.
Siltstone is similar to shale but consists of slightly larger silt‑sized grains. It lacks the pronounced layering of shale and usually breaks into blocks. Together, shale and siltstone make up more than half of all sedimentary rocks.
Sandstone derives from medium‑grained sand particles, typically quartz, feldspar, or mica. Its composition and grain sorting reveal the depositional environment. Well‑sorted, rounded grains suggest long transport distances, while angular grains indicate proximity to the source.
Both are mixed‑size detrital rocks, ranging from fine grains to large boulders, with finer material filling the voids. Conglomerates feature rounded clasts, whereas breccias are marked by angular, sharp edges—signatures of high‑energy, turbulent settings or steep slopes.
