The deformation of a material so that its layers move laterally over each other. Shearing bends, twists, and draws out rocks along a fault or thrust plane. A shear plane is the face along which shearing occurs. Shear strength is the ability of a rock or soil to withstand shearing; soil shear resistance is proportional to the tangent of the friction angle and therefore soils with larger friction angles exhibit a higher shear strength. Two major parameters of soil shear strength are cohesion, which is boosted by plant roots and peaks when soil moisture content is 12% to 14%, and internal friction, which is the resistance of a soil mass to sliding, itself inversely related to the amount of moisture in the soil. Slope failures are the consequence of loading conditions that exceed soil shear strength (Simoni et al. (2008) Hydrol. Procs 22, 4). Shear stress happens when the applied force acts tangentially to the surface of the body (the down-slope component). It is opposed to a normal stress (the slope-normal component) which is applied perpendicularly.

See Namikas (2000) J. Sed. Res. 72, 2 on measuring bed shear stress during aeolian saltation; Sime et al. (2007) Water Resources Res. 43, W03418 on estimating river bed shear. Within a glacier, the level of shear stress (τ‎) experienced at any point is dependent on the ice thickness and the surface slope. It can be expressed as:

where ρ‎ is the density of ice, g is the acceleration due to gravity, s is the surface elevation, z is the elevation of the point within the glacier, and ά‎ is the surface slope of the glacier.