Sediments are defined as naturally-occurring, loose grains of any composition or origin at the Earth's surface. Most sediments have a lithogenous origin - they form by weathering and erosion of any type of rock. Some sediments have a biogenous/biogenic origin - they are the remains, or fragmentary remains, of once-living organisms (animals, plants, microbes, etc.).

Sediments sizes:

Sediments range in size from incredibly small to immensely huge. In sedimentology, categories of grain sizes are given specific names - see the list below, arranged from the largest sediment grain sizes at the top to the smallest grain sizes at the bottom.

• Megaliths - 32 to 1000 km

• Monoliths - 1 to 32 km

• Slabs - 64 m to 1 km

• Blocks - 4 to 64 m

• Boulders - 256 mm to 4 m

• Cobbles - 64 to 256 mm

• Pebbles - 4 to 64 mm

• Granules (a.k.a. grit) - 2 to 4 mm

• Sand - 2 to 1/16 mm

• Silt - 1/16 to 1/256 mm

• Clay - <1/256 mm