Much of the subsurface soil of Greenland, Svalbard and the northerly regions of Scandinavia, Russia, Alaska and Canada has been frozen since the last ice age. Continuous freezing and thawing in the top 20 centimetres (7.9 inches), known as cryoturbation, allows meltwater to circulate and keep the soil relatively fertile. But lower layers than this remain permanently a few degrees below freezing, which can dramatically affect the landscape. Here, drainage is very poor, resulting in boggy ground dominated by mosses, small hills (pingos) pushed up by the ice and deep cracks where water can collect and freeze. Arctic areas with little permafrost (especially southerly forest tundra regions) are host to thick shrub tundra, willows, dwarf birch and other hardy plants.