Standing waterbodies can be divided into puddles, ponds, fish ponds, swamps and lakes. Puddles are shallow depressions filled with water that dry out, while ponds are depressions that do not dry out, and light penetrates to the bottom, enabling the development of aquatic vegetation. Swamps are shallow standing waters characterised by a low pH value. They are habitats for many species of plants, insects, fish, amphibians, reptiles, birds and mammals. Lakes are the largest and deepest standing water bodies. Most were formed by glacial action. Slow shifting of kilometre-deep ice sheets resulted in these depressions, which filled with water after the ice retracted or melted.