The onset and termination of the formation of debris cones and the rate of denudation of source areas were studied in the Langtang Valley, central Nepal Himalaya. A stereo photogrammetric analysis was employed to map a detailed topography of debris cones, through which the volume of 19 cones was calculated. Debris cones were classified into four types (types 1-4) based on the criteria, such as incision of a toe, sizes of a cone and surface boulders, altitudinal distributions and vegetation cover. Cones of types 1-3 had been formed after deglaciation following the Langtang stage (3310 years BP). Type 1 cones are characterized by incision of toes by glaciers of the Lirung and Yala stages. The formation of the type 1 cones terminated well before 2000 years BP, Radiocarbon dates of charcoal fragments and buried soils show the major period of formation of the type 2 cones from 3300 to 2000 years BP. The rate of sedimentation calculated near the toe of cone 3 has been reduced in the last 5 centuries from 30.0 cm/100 years to 5.5 cm/100 years. The large amount of denudation of source areas (116.9-132.5 m) indicates that slope failures initiated the formation of type 3 cones. The development of type 4 cones made less of a contribution to landscape evolution in the valley. The rates of denudation for source areas were calculated except for those above type 3 cones. The average rates are 15.6-6.2 mm/year (above type 1 cones), 3.2 mm/year (type 2 cones) and 3.4 mm/year (type 4 cones). Debris cones of types 2 and 3 are located on the valley floors which are completely deglaciated. Those of types 1 and 4, on the other hand, are located by the large valley glaciers with debris cover. The formation of type 1 cones is important especially in terms of debris supply to moraines and supra-glacial debris in the upper valley. (C) 1998 Elsevier Science B.V. All rights reserved.