The process in which sediment is laid down is called deposition. Deposition occurs when the velocity of a fluid (such as water or wind) carrying sediment decreases, causing the sediment to fall out of suspension. The rate of deposition depends on the size and density of the sediment particles, the velocity of the fluid, and the amount of turbulence in the fluid. Deposition is an important process in the formation of sedimentary rocks, such as sandstone and shale.