In some places, a distinct tree line separates forest from tundra. Soil particles in the tundra derive almost entirely from mechanical breakup of rock, with little or no chemical alteration. Inceptisols with weakly differentiated horizons are dominant. Continual freezing and thawing of the soil have disintegrated its particles. Like the northern continental interior, the tundra has a permanently frozen sublayer of soil known as permafrost.
Geomorphic processes are distinctive in the tundra, resulting in a variety of curious landforms.
Under a protective layer of sod, water in the soil melts in summer to produce a thick mud that sometimes flows downslope to create bulges, terraces, and lobes on hillsides. The freeze and thaw of water in the soil sorts out coarse particles, giving rise to such patterns in the ground as rings, polygons, and stripes made of stones. The coastal plains have numerous lakes of thermokarst origin, formed by melting groundwater.