The Beaufort Plain of northwest Banks Island is currently being dissected by a series of valleys which portray a striking asymmetry of their slopes. In most valleys aligned in a direction northwest southeast, the steeper slope faces towards the southwest. The importance of both fluvial and degradational processes is stressed in the interpretation of these valleys. The asymmetry is the result of differing microclimates existing upon differently exposed slopes, which favors the development of solifluction processes on the northeast facing slopes. Consequently, the stream moves laterally towards the slope producing the least colluvium. The southwest facing slope is then undercut and steepened in angle through the operation of fluviothermal erosion processes. The microclimatic differences on the two slopes can be related to the dominant westerly winds in this part of the arctic which (a) deposit snow preferentially on the lee sides of valleys during the winter and (b) promote evaporation from exposed slopes during the summer.