A slope is subject to the influence of gravity and possible pressure of ground water, which tend to cause sliding or caving. It is also subject to surface erosion from running water, wind, and alternate freezing and thawing, or wetting and drying. Weathering causes changes in particle size and composition. Bank slope stability can be attained by benching, by growth of vegetation, and by artificial protections, such as masonry walls, drainage systems to intercept or remove ground water, and fences to catch rolling pieces.