Gravity is a force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat surface, parallel to the Earth's surface the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting tangential to the slope.
The tangential component of gravity, gt, causes a shear stress parallel to the slope and helps to move the object in the down-slope direction.
Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object.
When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope.
Thus, down-slope movement is favored by steeper slope
angles (increasing the shear stress) and anything that
reduces the shear strength (such as lowering the cohesion
among the particles or lowering the frictional resistance.