Siliciclastic rocks, also known as terrigenous, detrital, or clastic rocks, are a group of sedimentary rocks consisting of fragments of minerals and rocks derived from pre-existing rocks. Such fragments are termed clasts, regardless of their composition and size. Clasts originate due to weathering and subsequent erosion of the rocks exposed on the surface of the Earth. Weathering, acting on rocks as physical and chemical alteration processes, disintegrates rocks to loose fragments (detrital particles) producing new minerals that are stable in sedimentary environments, like clay minerals. Erosion by agents such as wind, water, ice, or gravity, removes the products of weathering and transport them to the site of deposition. Deposition occurs where the velocity of the transporting current becomes too low, allowing clasts to settle to the bottom of a sedimentary basin. Clasts accumulated in a basin, called sediments, pile up layer after layer, increasing the weight, heat, and pressure, on the older, underlying sediments. As a result, sediments progressively compact at depth, expelling the water trapped in their pore spaces. The compacting sediments undergo several chemical and physical transformations (diagenesis) that bound sedimentary particles together producing lithified sedimentary rocks.

Classification of clastic sediments
Sedimentary processes (weathering and erosion) produce a variety of clasts with different grain size (= average diameter) that we classify according to the Udden-Wentworth logarithmic scale into four main classes:
– gravel (> 2 mm), which includes pebbles (2 – 64 mm), cobbles (64 – 256 mm) and boulders (> 256 mm);
– sand (0.0625 – 2 mm);
– silt (0.004 – 0.0625 mm);
– clay (<0.004 mm).

The scale divides further these classes in sub-classes (e.g. fine sand, very fine sand). These names can be used to refer to single clasts (e.g. a gravel clast) or a sedimentary deposit of loose particles with a specific grain size (e.g. gravel deposit or simply, gravel). The logarithmic scale arbitrarily sets the limits between classes in order to define standards, but some of these numbers are useful for classification purposes. In particular, the 0.0625 mm boundary corresponds to the limit of the human eye to distinguish objects: we can recognize very fine sand, but silt and clay particles are too small and are not recognizable in the field. For this reason, field geologists normally group clay and silt together as mud, comprising grain sizes that are smaller than 0.0625 mm.