A Multivariate Analysis of Clast Displacement Rates on Stone-banked Sheets, Cordillera Real, Bolivia Bernard Francou 1 * and Pascal Bertran 2 1 ORSTOM Ecuador, Ap. Postal 17.11.6596, Quito, Ecuador 2 Institut du Quaternaire, Universite de Bordeaux 1, avenue des FaculteÂs, 33405 Talence, France ABSTRACT Clast movements of stone-banked sheets were measured for ®ve years at Chacaltaya (5200 m ASL) in the Bolivian Andes (168S). This environment is characterized by frequent sur®cial freeze±thaw cycles and high displacement rates, up to 100 cm a 71 on the sheets and 30±40 cm a 71 on the stone banks. Multivariate analysis makes it possible to identify slope gradient, clast size, type of clast substratum (®ne- or coarse-grained material) and clast morphology as the main factors controlling the movement. The material in the sheet appears to move faster than the stone bank and the stone banks have a greater velocity than the stone pavement located in the front of them. As a consequence, downslope surfaces are progressively overrun by the banks. This analysis allows quanti®cation of the mechanisms which explain the strati®ed nature of slope deposits in an Andean periglacial environment. # 1997 John Wiley & Sons, Ltd. RE  SUME  On a mesure pendant cinq ans aÁ 5200 m d'altitude aÁ Chacaltaya dans les Andes de Bolivie le deÂplacement super®ciel des fragments rocheux aÁ la surface de couleÂes aÁ front pierreux qui geÂneÁrent un deÂpoÃt strati®eÂ. Dans ce milieu aÁ cycles de gel super®ciels et freÂquents, les deÂplacements moyens sont rapides: jusqu'aÁ 100 cm a 71 aÁ la surface des couleÂes et 30±40 cm a 71 sur les fronts de coule es. Une analyse multivarie e fait ressortir le roà le majeur qu'exercent la pente, la taille des eÂleÂments, leur support (®n ou grossier) et leur forme sur les vitesses de deÂplacement. Lacoule e apparaõÃt comme e tant plus rapide que le front pierreux, et lui-meà me plus rapide que les eÂleÂments du pavage situeÂs aÁ l'aval. Les surfaces aval se font donc recouvrir progressivement par celles de l'amont, ce qui con®rme les meÂcanismes qui sont aÁ la base de la stratogeÂneÁse dans le milieu peÂriglaciaire des Andes centrales. # 1997 John Wiley & Sons, Ltd. Permafrost Periglac. Process., Vol. 8: 371±382 (1997). (No. of Figs: 6. No. of Tables: 10. No. of Refs: 17.) KEY WORDS: strati®ed slope deposits; slope processes; quanti®cation; Bolivia INTRODUCTION One model to explain the formation of strati®ed slope waste deposits in periglacial environments concerns the mechanisms occurring in stone- banked sheets (SBS) (Francou, 1988; Van Steijn et al., 1995). The basic geocryological processes involved are needle-ice, frost creep/geli¯uction and the frost sorting of clasts (Francou, 1990; Bertran et al., 1995a). Previous studies in dierent peri- glacial environments have shown that optimal CCC 1045±6740/97/040371±12$17.50 Received 1 May 1997 # 1997 John Wiley & Sons, Ltd. Revised 15 October 1997 Accepted 20 October 1997 Permafrost and Periglacial Processes, Vol. 8: 371±382 (1997) * Correspondence to: Bernard Francou, ORSTOM Ecuador, Ap. Postal 17.11.6596, Quito, Ecuador. E-mail: francou@ glace.com.ec