Mud

Cohesive sediments are a mixture of clay, silt, sand and organic matter. Mixed in water they form what commonly is known as mud. This sediment is predominantly fine-grained and the physico-chemical characteristics of the clay minerals cause aggregation of these particles into flocs. Flocculation is furthermore enhanced by the presence of natural organic matter found in the aquatic environment, often of biological origin. Hence, cohesive sediment particles in general are not solid particles but flexible flocs which can aggregate or break-down, depending on various external factors (such as turbulent shear, concentration and salinity of the ambient water). Their size, structure (i.e., their density) and strength are thus variable, much in contrast to non-cohesive particles like sand and gravel.

[Hydraulics Laboratory: Cohesive Sediment Mechanics]