The regional average basal shear stress &tgr; of a thrust sheet of thickness H is equal to the down-surface slope stress pgHα. Thrusts always move in the direction of surface slope α, even if they are moving up the dip β of the base. The sole thrust beneath the Canadian Rockies had &tgr; of the order of 5¿106 Pa (50 bars). Listric normal faults in the main ranges coincided with a reversed sens of &tgr;. A dimensionless number gives the relative magnitude of compressive surface to gravitational forces in the formation of a thrust. The general strength of the rock imposes severe restrictions on the magnitude of surface forces, and gravitational forces dominate in the emplacement of entire thrust sheets, although compressive surface forces are important in the toes. If gravitational forces are dominant, continent-continent collision is not required to produce an orogeny, but signifcant surface slopes are. An importnat cause of such slopes is rapidly uplifted magmatic arcs. Piggyback stacks of imbricates can be interpreted in terms of the evolution of paleoslope. Ophiolite thrust sheets have affinities with thrusts made of shelf and slope sediments and may be treated geometrically and mechanically in similar ways.