This image, taken from an altitude of about 3,000 m, shows an esker ridge superimposed at almost right angles on a drumlin, indicating that the glacial ice in which the esker was deposited was not active at the time of its formation. Note the frost polygons on the surface on the left side of the image and the concentration of trees in low areas where there is adequate moisture and protection from wind. This site is south of the tree line (the boundary between Boreal Forest and Tundra), but isolated stands of trees can be found in similarly moist and wind-protected environments 100 km or more north of the tree line. The frost polygons indicate that the area of the photograph is underlain by permafrost.