In the coastal zone we find sediments like quartz (SiO2), carbonates and clay minerals (sheets of silicates, see Intermezzo 2.3). Depending on the particle size we distinguish silt and clay, sand, gravel, and cobbles (see Sect. 2.6.2). Clay particles are very small with a large surface area compared to their volumes. This surface area is chemically active, which, especially when wet, leads to the typical cohesive characteristics of its bulk form. Quartz and carbonate sands on the other hand are non-cohesive; the grains do not stick together. A handful of pure sand cannot be picked up by hand in the way a piece of clay can be picked up. In these lecture notes the main focus is on sand.